I have another similar problem that I can’t figure out. I have no audio over HDMI when running SNES in NTSC mode. Neither a supercic modded PAL machine running in NTSC mode or a supercic modded NTSC machine running in NTSC mode have any sound over HDMI. Both machines have sound in PAL mode how ever.
I’ve since updated the firmware from what the OSSC came with (can’t remember which version) to the latest 0.81-aud and same thing… no audio over HDMI in NTSC mode while PAL does.