Really only one destination you should care about https://www.hvsc.c64.org/downloads.
You can get the full collection of .sid files OR wrapped .sid files in PSID64 format (.prgs) (better compatibility).
Look for a link called "HVSC79_PSID64_packed.7z" (for HVSC79) or something like that.
chipsynth C64 ships with the open source kernal, basic and chargen replacements from
https://github.com/MEGA65/open-roms.
Those are amazing, but sadly not 100% compatible with all routines compared to the stock roms.
So it's not that the CPU core has issues (I ran all possible tests on it including illegal opcode tests)
more than the routine assumes a stock C64 romset and throws bad opcodes at the emulated 6510.
Due to copyright reasons we cannot ship them, but if you have those ROMS already
(I need to legally add that you should own a real C64 for this),
then place those in the "Data" folder and restart.
ALSO!:
The SID player itself is code that lays on top of the C64 emulation environment and some SIDs might
not play right, or at the right speed because of the way our player works. Please try the PSID64 version (see answer 1)
and if that fails as well, we have a problem.
See this post in the forums for the current list.