@gallina - You can often see swelling at the top of bad caps - see for example http://www.robotroom.com/Faulty-Capacitors-1.html .
Your problem could also be a loose connection. I’ve had problems with the spade connectors inside the speakers corroding or coming loose (but it’s hard to access those unless the adhesive holding the speaker cases together has also deteriorated to the point where you can open the cases). Maybe you can try wiggling the speaker cables to see if that has any effect.
It could also be a loose or broken connection on the audio input.