Why doesn't my Super Famicom have any sound?

I'm have an old Super Famicom (I'm in Japan), and while the games don't always work on the first try, they do work. Last night I got Super Mario Kart to work fine, and I was playing it for an hour or so.

This morning however, when I turned it on, the game had visuals but no audio. I did have to eject it and push it back in a few times to make it play. I got a few other games to play, but none of them have sound.

This is my girlfriend's old system, and it's been sitting in her parent's house for at least 15 years or longer. She thought it was broken, until I got it working last night.

I have two RCA cables and two AC adapters. I don't know how old they are, because they were with the snes when I found it. They are all Nintendo licensed. I tried both sets of RCA cables, and both display visuals, but no audio. I'm not sure if the console just broke over night or what, but I would appreciate any help I can on diagnosing this problem.

I looked at the bottom of the console, but I think I need a special kind of tool to open the it up. I'd rather not do that unless I really have to. Thanks again for anyone who can offer some insight.