Blown speaker, probably. Talk radio turned up kinda loud will do that. So much distortion, the speakers can't handle it. Easy to test. Door panels come off lickety-split. Speakers easy to remove as well. Good opportunity to upgrade. Can you borrow a known-good speaker for testing? Doesn't have to be top-notch, just known good.
Could be a dirty control, but not likely any more. Radio Shack should have some contact cleaner. You don't have to disassemble the radio (and this should NOT be the first step), just spritz the control from the outside.
Whoops, mixed up my posts. "Blasting it" through factory speaker will certainly blow the cones. So much distortion by the low-powered amp will kill speakers faster than too much power.