Intermittent sound from stereo

I have a 2012 Prius that I've had for 9 years. About 8 months ago, I started losing sound from the stereo. The display screens works fine, but no sound. Sometime the sound will be out for a few weeks, sometimes only a day. Sometime the sound will work for a month, sometimes only a day. It varies greatly and will start working randomly. I did notice that when it starts working again, it's a fade in. I have a Kenwood KAC8405 amp, which seems to be working, all of the lights are on anyway. I also just upgraded the speakers last week, but that didn't help. I have checked the connections at the speakers and at the radio head unit. Any ideas?