Thanks for the responses. I am looking for any typical failure mode for this system. All equipment is original. There has been no abuse, water damage, or anything else that I know of. I doubt both front and rear speakers on same side would go together. I have had several of these cars and no issues before so not much to go on. I also don't see much online about similar conditions with Vibes losing one side's speaker volume... or at least the low frequencies.
I saw a suggestion online about trying the CD player, and if the speakers works the head unit is the issue, but when I went to load an audio CD it just got spit out again. Not sure that was ever used in the 5 or 6 years that we owned the car.