Do both speakers quit at the same time? If not, suspect the speakers. If they do, it's typically a radio problem unless your system uses a remote amplifier. Unfortunately GM chose to build in a lot of "got'chas" to cost you money after the sale and their audio systems is one of them. They stopped allowing us to buy radio service manuals after the '94 model year because they want all the repair business for their two grossly over-priced repair centers. If the speakers are quitting at different times, I would try one replacement first and hope that one doesn't act up. If both still do, it's a very expensive trip to the dealer.

GM also figured out that if they build the Body Computer into the radio, you can't remove it to install an aftermarket replacement. You have to get it fixed. One of the systems they built in is the warning chime.