There's a Days Gone sound bug that's hard to miss if it affects you. There a few teething troubles in Days Gone at launch but audio problems were one of the most common issues. While not exactly game breaking, things like having a completely silent motorbike can be immensely disorientating and have a big impact on you experience in the world. The bike glitch is the most common, but it's not the only one with things like Deacon yelling for no reason, or your weapons firing without any noise also affecting some people. It all depends on how this weird Days Gone sound bug presents itself.
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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 a future patch.