Uploaded the test song for my sid + fm expander cart driver, giving 12 channels. As the cart is quite rare (though officially released by Commodore) probably best way to hear it is through the Vice emulator. (see my note in the comments on setting that up)
The driver allows my sid part to be replaced with another player (eg: GoatTracker) so 2xsid is possible and working, possibly 3xsid (for a ridiculous 18 channels on one c64) but I haven't tested the latter one as yet. Beta testers have my clunky XM converter to test out so hopefully somebody will come up with a longer track before I work on the next version.