This is quite weird. Please email an example of a SID file and player you are trying to use that play fine on a real C64, but are not running on your N4DDR board. I'll try them on my MEGA65 here, and let you know the result.
In the very latest bitstreams on the feature branch I am working on, there are 4 sids at $D400, $D420, $D440 and $D460. They are repeated after that to $D4FF, and then repeated in swapped left-right order at $D500, to try to help some old dual-SID programmes to work -- although they tend to be a dogs breakfast.
For this reason, we actually added support for simple playing of SID files to the m65 utility, which fixes the addresses of SID files it loads. But also expect that we will add more SIDs if we have space in the FPGA at the end, possibly 8 or 16 in the end, so software written for the MEGA65 should behave and use only the primary images of the SIDs at $D400-$D47F.