Are you trying to play some sort of proprietary "adlib sound format" in DOSBox? If not, understand that you can just record to a plain WAV format file and listen on any soundcard (except, ironically enough, an adlib -- though I did once see a special driver that got some digital audio out of an adlib somehow, only it seemed to need to use 100% of the CPU to do it.) d-:
Anyway, emulation of an adlib is achieved simply by reproducing the way the adlib itself actually produced the sound. Essentially creating a software version of the actual hardware itself.
That said, if it's that you are trying to play a proprietary format, DOSBox may not be your actual best choice here. There are actually a number of tools out there -- primarily various "game audio player" type applications which can play such formats. In particular, you may want to look through plugins for Winamp or Foobar2000 (or XMMS or whatever else you may use.) You may be able to find something that can play such files with less resource wastage and to convert it more directly to WAV format. EDIT: A quick glance found this for Winamp: http://www.winamp.com/plugins/details.php?id=10240 May not be exactly what you want, but, I think if you keep searching you can find something to suit you better than trying to record from an emulated DOS system.
Can't imagine why someone would WANT to listen to adlib though. I'll tell you one thing, I may kind of miss the days of things like the Sound Blaster 16 and AWE or the better cards, but, one thing I do not miss would be the Adlib card I accidentally fried. You may want to dig around and see if you can't find MIDI versions of the same music. When played on an adlib, it should sound like it originally did, but, more importantly, when played through a quality MIDI synthesiser (I particularly recommend the Yamaha XG synthesis if you don't want to deal with soundfonts on a soundblaster card. The MS synthesiser isn't really that great, even if it is supposed to be a ripoff roland emulator, but, then roland GS is considerably older than Yamaha XG.) I actually still occasionally listen to MIDI files going back and forth between the Yamaha software XG synthesiser and my soundcard's sythesiser loaded down with something like 40MB of soundfonts (I can't afford those commercial soundfonts, so 40MB is pretty good for all public domain stuff.) Officially MIDI is dead, but, unofficially you can still do some nice stuff with it and some professionals still use it to some extent -- albiet not just simply a MIDI file played through a synthesiser and recorded, but a lot more complex.