MP3, AAC
Winamp > Ctrl+P
Plug-ins > Input
double click 'Nullsoft MPEG Decoder'
Decoder tab > uncheck 'Allow stereo output'
close out of prefs
OGG
Winamnp > Ctrl+P
Plug-ins > Input
double click 'Nullsoft Vorbis decoder'
Multichannel tab > checkmark 'Downmix all to mono'
close out of Prefs
WAV
Winamp > Ctrl+P
Plug-ins > Input
double click 'Nullsoft Waveform Decoder'
checkmark 'Downmix to mono'
close out of prefs
MOD
Winamp > Ctrl+P
Plug-ins > Input
double click 'Nullsoft Module Decoder'
Mixer tab > uncheck 'Stereo Output'
close out of prefs
MIDI
Depending on your preference...
Method #1
Winamp > Ctrl+P
Plug-ins > Input
double click Nullsoft MIDI player
Device tab
under the Device: drop-down list select 'Direct Music / Microsoft Synthesizer (with output)'
close out of Prefs
Then use one of the "Mono Playback Via a DSP/Effect Plug-in" suggestions below.
Method #2
Winamp > Ctrl+P
Plug-ins > Input
double click Nullsoft MIDI player
Device tab > select midiOut device or something that closely reflects your soundcard
Sampling tab > checkmark 'Sampling Enabled' > under 'Format' select 'Mono' from the middle dropdown box
close out of prefs.
It may take some trial & error with settings. You also may want to try a different device in the Sampling tab.
Note: Changes may not take effect until playback is restarted.
Audio CDs
Winamp > Ctrl+P (for preferences)
Plug-ins > Input
double click 'Nullsoft CD/LineIn plug-in'
checkmark 'Enable digital audio extraction when possible' or/and 'Use Sonic engine when possible'
close out of Prefs
close & restart Winamp
Then use one of the "Mono Playback Via a DSP/Effect Plug-in" suggestions below.
Other Formats or Third Party Input Plug-ins
It will depend on the input plug-in for the file type. Check the plug-in's config for a option similar to disable stereo, down mix to mono, mono playback, etc.
Otherwise, try one of the "Mono Playback Via a DSP/Effect Plug-in" suggestions below. However, actual results may vary with the Input plug-in or some Input plug-ins may not work (work properly) with DSP/Effects plug-ins.
Vh
Hr_hvalsoe