IIRC, I used this over a decade ago, and probably made some skins and used ResEdit to customize MPlay. Although the interface is rather plain, this customization (or using the currently downloadable skins) is something that was attractive to Mac users.
Eventually I bought SoundJam (currently iTunes), Audion (the best), and Unsanity Echo (promising but discontinued when they focused on haxies).
While there are alternatives that I use like Vox, VLC, and others, MPLay has these (subjective) advantages; interface and mixer.
On the downside, opening the Trackmixer increases CPU usage considerably. Apparently there's no manual. When the player is stopped Quick Reference is available under the Tools menu and can be launched as a separate application.
This looks and feels dated, but still is a fairly good mp3 player for people looking for something different, particularly the mixer feature. It's not something I would purchase, but can't complain with the current price.