I've seen a transistor amp IC similar to that (linear amplifier, class AB topology) fail in a similar manner. In my case it was a Sanyo STK amplifier chip, the amplifier chip ran very hot indeed but still worked ok, but after about 10 minutes the left channel would go very distorted. I suspect the high side transistor for the left channel had become leaky, such that as it got hotter and hotter it got leakier and leakier. This led the output voltage to slowly climb, leading to more dissipation in the speaker and STK chip, though it still retained some modulation. It didn't seem like that amp had any DC protection so presumably the speaker could easily be destroyed and/or cause a fire should the amp chip continue to fail in this manner.