All archive plugins:
Make contained files visible to the Input plugin as if the archive were a folder. This way shared, soundbank-type files are available to make the separate tracks playable in the archive. This already works with MINIPSF/MINIPSF2 and PSFLIB/PSF2LIB files. It does not work with MINIGSF and GSFLIB files, for example.
Archive contents are visible to native plugins (eg. the MIDI plugin can load an SF2 soundfont from within the same archive as a MIDI file), but unfortunately that isn't possible with Winamp plugins. It could be achieved by extracting all of the files to disk, but that would be slow and wasteful (there's nothing to say that the other files are wanted).
Ian, would it be possible to have an option to modify handling of natively supported filetypes (WAV, MP3, OGG) to skip using XMPlay's replay engine for files with a different extension than the usual?
Here's something to try... www.un4seen.com/stuff/xmplay.exe
Rather than a "skip" option, each plugin has its own "priority filetypes" list. If any plugin has a file's extension in its list, it will get 1st try at the file. If multiple plugins have the extension, they will be tried in the order that they are shown in the "Input" plugins options page. If all priority plugins have been unsuccessful, the usual order of things follows (ie. built-in decoders, then the plugins).
Mono output has been in-built for as long as I remember (Options & stuff -> Output -> Channels).
Yes but it does not seem to work in mono, at least with my current software.
Check that you have also enabled the "Downmix multi-channel" option.