I'm actually an audio engineer and loudspeaker designer. But I don't drive this Volt; the person who does just listens to the radio as background. So we did not upgrade the sound.
That said, I think the sound is pretty decent. Yes, there is something missing in the lower mid bass, and the clarity is not the ultimate.
It is quite possible the "upgraded" stereo tries too hard to impress and is boosted/bloated.
It's also VERY possible, if you listened to a test drive model, that the tone controls were all boosted to the max, which makes everything sound like [email protected]
You should take some CDs of music you like, or better yet load select tracks to your phone and be sure to take an 1/8" to 1/8" AUX cord. Go in to the audio menu, be sure everything is set to flat except the fader should be several notches forward to minimize the rear speaker sound.* Now listen while stopped and while driving, to a base car and Bose car.
There is also a setting (I forget the name) which appears to be speed dependent volume. I'm not sure if this also changes equalization dynamically.
*You don't listen in stereo at home with rear speakers, do ya? So why would you in the car? It's just a historical accident, turn that nonsense from the back down! (True surround sound is a different story, but not offered in the Volt)