It's the Bolt Bluetooth volume
With data from 2 devices, and iPhone X and a Pixel XL, I feel confident in saying that it's the Bolt that for some reason plays sound via Bluetooth orders of magnitude lower than from any other source, including from the same devices connected via CarPlay/Android Auto. with both devices, now, I've been playing music that was stored on the device via CarPlay/Android Auto, disconnected the cable, and selected the sound source as the device via Bluetooth. What once was a reasonable volume becomes barely discernible--and that's with the car parked in a quiet garage.
On another thread, someone claimed that the volumes are set separately, that you can adjust the Bluetooth volume only when Bluetooth is connected. I disagree. Multiple times I have increased the volume while playing from Bluetooth and switched sound sources, only to be practically blasted out of the car.