Some observations from my end, which may prove helpful...
- 'No sound or chimes from HU' -- this happens to me on occasion, but even with an aftermarket radio installed (with appropriate module to enable chimes, etc. Because of this, I've started leaning toward an intermittent bus issue, which may be difficult to track down, at best. BTW, the chimes are actually generated from the OEM head unit, which is why an aftermarket module is needed if the HU is replaced with an aftermarket one.
This 'no sound' problem goes away after I've arrived at my destination, shut off the vehicle, exited, and restarted (in other words, it happens so infrequently that I forget to test if a KOEO alone will 'fix' it. One of these days I will remember, but since I can't duplicate the problem at will... )
Also, since it 'goes away' on next restart, and most of my trips in the Envoy are fairly short, it's not a major concern to me, TBH. Prolly another reason why I keep forgetting to try and isolate it further, when it happens.
Finally, I've noticed more people reporting this 'no sound' / no chimes issue in the last few months. What I don't know yet is what percentage of owners will eventually be affected. It's not 'everyone', or we'd have heard more on this, by now. Something in the bus network or the HU degrading over time, is all I have, 'for now'.
- 'The amp'... After having 3 different vehicles with the Bose amp (similar vintage; two Sierras and an Envoy), and all of them exhibiting (different) problems, along with the same issues in another one that I got from a member here (not a scam issue; just relaying that it eventually broke as well), along with many other posts from people here also relaying similar issues... I've deduced that these are an ongoing issue as our vehicles age. Thankfully, this can be resolved by replacing the amp -- either with an aftermarket unit, or by using the internal amp of an aftermarket HU / receiver, if that's sufficient for your needs.