Hi,
I had noticed the same problem but had not found the time yet to investigate... Now I saw your post and after some testing I found the solution
The new sdl_mixer uses libmikmod for MOD files, and libmikmod is not included in Slackware.
The solution is the following:
1) get libmikmod from SlackBuilds.org, build it and install it
2) get the sources for sdl from your Slackware mirror, build it with
Code:
BUILD=1_alt ./sdl.SlackBuild
3) upgradepkg the new package (it will be in /tmp)
4) Ready to play supertux with sound!