I have a similar problem. I found my old N64 and I cleaned it and in worked but no sound at all. I checked the cables with a multimeter and they were fine… so it had to be the board. In my research to fix it I have found usefull info that I haven’t tryed mysef yet, but it might help you:
* the capacitors that go through the audio might need replacement
** If you buy a HDMI ultra mod this bypasses the capacitors and process the video and audio on its own
** If HDMI mod is too expensive replace the capacitors