A couple of things may be going on. First off, the Spectra has a well known issue with leaky capacitors and several of them are quite near the audio section and cause audio issues. You indicated that the shop indicates that the radio checks out fine, but you may want to confirm that they have replaced all of the known leaky caps, even if they don't appear to have leaked just yet (they've probably leaked, but not visibly so the damage may still be happening to the boards).
Second, a Spectra is wide-band only (Astro-Spectra may be narrow-band or not) and your agency may have gone narrow-band, which would make the audio sound quiet on the Spectra. If this is the case, your radio may no longer be legal to use under their license and could be causing issues with them or other agencies that are close to their frequency. You may be forced to get a new radio to remain on their system (and nearly certainly will after the narrow-band deadline passes).