The Days Gone sound bug is quite prolific. Though there are a fair few rough edges knocking around in Days Gone, the audio is possibly the most noticeable and consistent bug you’re going to encounter. The noise cutting out and leaving your motorbike mute – without its trademark hardware purr – can be quite irritating. On top of this, Deacon may just end up shouting for no reason (we feel you buddy!) and your weapons may end up making no noise when your fire.
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Sony Bend is at least aware of the various sound problems and stated that audio drop out is a "known issue" and that it's looking into "a resolution to this". In the mean time one fix to sound dropping out or being muted is to "equip the default exhaust on to your bike".
It's not clear if that's just for the bike related problems or in general but it's definitely worth a try before you quit and restart the game. To do that you'll need to visit one of the survivor camp mechanics and change the exhaust under the 'Performance' tab. It's not clear how this helps if you've not reached a camp yet, although that happens within the first hour of the game. And, obviously, if you've not bought a different exhaust you might have to just to be able to change/equip the default option. It's a step towards a fix at least and hopefully the issue will be fully resolved in the next patch.