Change Winamp settings as follows:
In Winamp type Ctrl+P (for Preferences) navigate to > Plug-ins > Input > double click 'Nullsoft MIDI Player' > Device Tab > select from the Device: drop-down menu, then select: "DirectMusic / Microsoft Synthesizer (with output)"
DirectMusic / Microsoft Synthesizer (with output) uses software emulation of MIDI instruments instead of using your sound card's built-in MIDI support. This effectively allows Winamp's volume control for the playback of MIDI files instead of altering the system's MIDI volume, MIDI device volume or the Winamp volume in the Windows mixer (in Windows 7 anyway - have not tested this in other Windows versions)
Change the 'Device:' to "DirectMusic / Microsoft Synthesizer (with output)":
To configure Winamp to NOT lower the volume when (or after) you play midi files, try changing 'Volume Control:' to none:
Prefs (Ctrl+P) -> Plugins -> Input -> MIDI Player plugin -> config:
Device tab:
After changing the 'Device:' to either your sound card's hardware MIDI synth or to one of the DirectMusic software synthesizers (requires DirectX 8 or higher), then...
Volume Control:
Default = autodetect
You can change this to None
and then Winamp will never adjust the volume
Original Source: http://forums.winamp.com/showthread.php?t=320434
