ExoticA is an Amiga and retro gaming/computer music interest wiki including search-able computer music collections, game and demo scene information and plenty more.
I was wrong, the Sonix (not Sonic!) program used the *.smus extension. As for *.mus, I actually had only one such file, an obscure Adlib-format. According to Michael Doering, some Rob Hubbard 2 modules use this extension. All my RH2 mods are named *.dum for pattern data and *.ins for sample data, though.
In my collection there are now 133 formats, some more if you split up *.mod into ST, NT, PT etc...
And since I changed my mail addy I can't login to this forum... I mailed the most venerable administrator, so now I just have to wait...