I found a WinAmp COM Object for scripters who are familiar with COM and are interested in a bit more control than any dll I've seen thusfar. With it, it would be fairly simple to build in support for both /splay (using Windows Media Player) and $com (using WinAmp) into your mp3 player, giving your users the option of which player they wish to use.

If anyone is interested, I can write up some scripting examples of how to use that COM object to equate its methods/properties as they relate to /splay, $vol and $insong (plus the additional toys for the equalizer and preamp), and perhaps some examples of how one might deal with the built-in playlist stuff. None of it is all that hard, but if you're not familiar with $com, examples can help you see how to convert that page's methods into mIRC scripting.