That’s surprising, I’ve got a 2015 M70-C3 (exact same series, just a larger size), and I’ve been quite pleased with its compatibility.
I don’t think TV compatibility should affect audio, are you able to get sound from anything else? At least on a stock OSSC, the 3.5mm jack is a direct passthrough of the SCART port’s audio; the OSSC doesn’t even need to be powered for it to work. Is it possible that there might be an issue with the SCART cable or your HDMI audio mod?
On the video side, how often are you seeing drops with your SNES, and on what firmware? I’ve very occasionally seen some for x3 and x4 with 0.76 on my M70-C3, but they were on the order of less than once per hour.