For Monkey's Audio I've always used the MACDll_399t2.zip plugin.

SNESamp and Ganbatte! allow you to play .spc, .d00 and .d01 OPL2 files respectively.

Errius wrote on 2024-02-14, 18:12:

For Monkey's Audio I've always used the MACDll_399t2.zip plugin.

Thank you very much! I still need to find a flac plugin for winamp. That one from WinampHeritage site makes my winamp crash unless I uninstall it.

Garrett W wrote on 2024-02-14, 18:53:

SNESamp and Ganbatte! allow you to play .spc, .d00 and .d01 OPL2 files respectively.

Thank you as well. I will add to my collection of plugins 😀

Is there a list of all Winamp plugins anywhere?

ETA: These are the ones I have:

• File_Writer_output_plugin_v1_11.exe – Output plugin that writes audio to disk.
• in_mpc_0.99g.zip – Input plugin that allows playback of MPC files.
• Jump_to_File_Extra_v0_96ff.exe - Jump to File Extra (JTFE)
• MACDll_399t2.zip – Input plugin that allows playback of Monkey’s Audio (APE) files.
• ml_sony_v1.0.4.zip – This apparently allows playback of proprietary Sony audio streams.
• mp3g_plugin.exe – CDG Plug-in for Winamp. This displays lyrics. Intended for karaoke.
• nezp0948.cab – NEZplug. Allows playback of NSF/KSS/HES/GBR/AY files.
• Nulsoft_WMA_Input_Plugin.exe – Allows playback of WMA files.
• oldsk00l.exe – This is believed to be for playback of chiptunes.
• Tara_v1.0.3b.exe – Allows playback of RealMedia (RA) files.

more:
in_opus.dll - opus file decoder plugin for winamp 2 by Raymond Gillibert
in_usf.dll - 64th note (N64 music player)
in_msx.dll - MSXplug (player for KSS, MBM, etc.)
in_adlib.dll - AdPlug Winamp plugin v1.8
in_sap.dll - Atari POKEY music player
in_vorbis.dll - Ogg Vorbis player (maybe this one is included with Winamp, I forget)

Since this thread is showing up in search results, I should probably elaborate on some of the existing responses I had trouble following. I hope the amount of effort I've put in excuses necro-ing the thread.

• I couldn't find a MACDll_399t2.zip that contained more than just the bare Monkey's Audio DLL, but MAC_399F.exe from the Wayback Machine's archive of earlier versions of the Monkey's Audio site does the job.
• flac-1.2.1-win.zip from the FLAC SourceForge project contains what I believe to be the last version of the upstream plugin and, unlike other FLAC plugins I tried, it won't cause my WinAMP 2.95 on Win98SE to crash on startup... however, this hydrogenaudio thread says it has a bug which manifests when trying to play 24-bit files so, if you use those, maybe try 1.2.0 instead.
• Here's the Wayback Machine link for in_mpc_0.99g.zip in case you also have some Musepack files to complement your Monkey's Audio files.
• I had trouble finding an MO3-compatible MOD plugin that would actually work with the combination of WinAMP 2.95 and Windows 98SE without KernelEx. I wound up settling on this one.
• The Wayback Machine archive of in_gym.dll's website, including source, is here.
• If you want a curated bundle of plugins, or you're just having trouble finding a few, check Nonoo’s classic Winamp pack. That's where I found some I was having trouble with, such as a working version of in_wv.dll.
• If you want something not in that pack and it's not on winampheritage.com (which seems to have the best SEO), check modland and the plugin list on the Winamp page on the Video Game Music Preservation Foundation Wiki.

Here's a listing of the WinAMP folder in my Samba share for installing retro PCs, with hyperlinks for easy retrieval and original filenames and md5sums so that, if the links die, you have extra tools to find new copies:

48ce60e0c46d5aeee85faf59993e31fd 64thv12b3.zip (USF)
461d6dd074a10ecd250e694944173f83 adplug.db (update for in_adlib-1.8.2.zip)
f16534596deeae286f14cd5ed9cab3b9 Combined-Community-Codec-Pack-2007-02-22.exe
8e30390964da11cb88ec704769fb6759 flac-1.2.1-win.zip
056e6ddebf163eba21bce86c17905161 HE209.zip (PSF/PS2)
96796725ca48c97b0e6a65c7b8fe632d Highly_Advanced_(Winamp)_v0.11.zip (GSF)
a697101d59e213e25f4c92f05ce0a9bc in_adlib-1.8.2.zip
34fa006fe0a2cd59bb189fc48c142751 in_bass.exe (MO3)
ce7d48227ab11ed4efcf1dce2611a740 in_gym(0.1.6.4).zip (GYM, VGM)
a6815a2235e1eeb208d2c9f9fd42c6df in_mpc_0.99g.zip (Musepack)
94b3ab1b9f187754eba2c391bc8f0ef1 in_sap22.exe
e15e6133f4085a3143351d6cd8e1e9e9 MAC_399F.exe (.ape)
65c06368490d61d96d0a21898cdf1cf7 NEZ_Plug_v0.9.4.8.zip
7542c9560c7ca2b70f898b6a098392bd nonoos_classic_winamp_pack_v1.5.exe
↳ 3817a1909c453628ce7c1f7f5673527a Plugins/gen_hotkey.dll
↳ 7e4d7da1d7f7afabba75e1674ced661d Plugins/gen_timerestore.dll
↳ 89f0ed6f4bb30c430db8d5d3ec01af55 Plugins/in_mp4.dll
↳ 369e677b439888583326f1eb81c3e7df Plugins/in_sid2.dll
↳ 845c05e8d9eab1f6800b96af619eafcf Plugins/in_speex.dll (untested)
↳ 88cb779a918f7840ba14eb433d424021 Plugins/in_wv.dll
395421620bd9bd6a73a0c166ceff0f88 Samp334.exe (SPC, RSN)
8b07ceb9f2948ac95503b389be19b7ba VGM_Input_(Winamp)_v0.40.9.zip (VGZ)
1f19a5f196d8d79880d9b66d160034b6 winamp295.exe

NOTES:

• The USF plugin I'm using will cause WinAMP to crash on exit if a USF file has been played. Suggested alternative versions are welcome.
• The Plugins folder in the list is the subset of nonoos_classic_winamp_pack_v1.5.exe that I'm using.
• The Combined Community Codec Pack includes Media Player Classic, but it's on this list for its DirectShow filters.
• You can turn on support for more formats than are on by default by right-clicking in the FFDShow video and audio codec configuration GUIs provided by the CCCP and choosing to enable all stable formats and then adding the relevant extensions to the Nullsoft Directshow plugin's configuration dialog, but I make no promises about whether you'll have enough CPU to actually watch them. That's why I have in_mp4.dll for audio-only playback.
• The Nullsoft Wave plugin didn't like some of the .wav files I dumped from game resources. This can be worked around by renaming in_wave.dll to in_wave.bak, adding ;.WAV to the end of the list of extensions in the Nullsoft DirectShow plugin's configuration, and, if necessary (.wav is a container format), installing the Combined Community Codec Pack and using its FFDShow audio config GUI to enable more formats.
• The only VGZ-capable VGM plugin I've found so far resulted in "() -" titles in the playlist in what I assume is an "If there's no metadata in the VGM, don't fall back to the filename" situation. Leave the "disable VGM" checkbox in the GYM player unchecked for quieter playback but proper titles.

I believe that, aside from 2SF (Nintendo DS rips), which I can't find a Win98-compatible plugin for, that covers every format I'm used to playing with Audacious Media Player on my Linux PC. (It's basically an open-source "your choice of Foobar2k clone or WinAMP clone" for all major modern OSes, with an ffmpeg/Linux kernel-like policy of "bundle all the plugins". The three notable exceptions are USF and RSN format support and the ability to play more than just the audio when you drop a video file in your playlist.)

UPDATE 2024-06-02: Added plugins for GSF, SAP, and VGZ. Switched from spca601.zip to SNESamp to gain transparent support for playing .rsn files. (RAR'd albums of .spc files). Figured out workaround for problematic .wav files.

TODO: See if I have any Shorten (.shn) files anywhere and, if so, find a WinAMP plugin for them. Find a reason to care about Opus files and track down a suitable plugin.