The pin connector felt loose on my Twin Famicom so I decided to open it. Managed not to bend or abuse any parts while doing so. Noticed nothing wrong and put the cover back on, after which the sound on the attached video appeared. Tried a few different RCA cables but the result was the same. Since then it's become less noticeable but is still there. What the hell happened?
Have you tried adjusting the mic volume on controller 2?