First you need to make sure the link between DOSBox and the MPU UART (the interface provided by the USB adapter) is configured.
https://www.dosbox.com/wiki/Configuration:MIDI

The fact that he is hearing something even with the MT-32 powered off would mean that Dosbox is using the built in GS emulation in Windows. So yes, like was stated above, you need to make sure that Dosbox is configured to use the correct midi device because it seems to be currently using the default GS emulation and that won't sound correct on any games that really used the MT-32.

Finally i get my MT-32 running and playing. =) Thanks a lot guys. I've tested it with King's Quest IV and Wing Commander and sounds incredible.

The issue was that i used a custom build DOSBOX version and the MPU-401 emulator doesn't work. Finally I decided to run the official DOSBOX version, configure the MIDI settings to UART and select the proper device and that's it.

The only remaining issue is that i hear an annoying little buzzing sound and the MT-32 doesn't display the midi message for certain games. But i think it is caused by the generic AC-Adapter and the poor quality USB-to-Midi cable, so i will replace them in the near future.

Anyway, i've enjoyed every note and instrument played on my MT-32 and the best of all is i've only paid $45 on my country ebay clone page. =)

Thanks a lot guys, i will post some videos and images as soon as possible.

Those cheap USB adapters are known to have issues with long SYSEX messages, which could be causing problems with the LCD message and potentially with custom instruments as well.

Yes I use the expensive but worth it UMONE in my setup. I also have the midi settings at Intelligent, but have it set for win32 since I have the UMONE set as my default midi device in windows. I would also advise making sure you set the option in the Midi section for the delay sysex option to help as well. I can't recall right off hand exactly how that is worded in the config file, but yeah.

cabezonnor wrote:

The issue was that i used a custom build DOSBOX version and the MPU-401 emulator doesn't work.

Which one, if I may ask?

Srandista wrote:

cabezonnor wrote:

The issue was that i used a custom build DOSBOX version and the MPU-401 emulator doesn't work.

Which one, if I may ask?

The SVN build. from http://www.ykhwong.x-y.net

Well yeah, Daum-SVN it's really old build.

cabezonnor wrote:

Srandista wrote:

cabezonnor wrote:

The issue was that i used a custom build DOSBOX version and the MPU-401 emulator doesn't work.

Which one, if I may ask?

The SVN build. from http://www.ykhwong.x-y.net

Interesting as that is the build I still use most of the time. It used to give me constant issues with my mouse jumping but a change the video render took care of that issue for me and so I still use it. I believe my build of Daum-svn is the same 0.74 as the latest Dosbox release...