Chances are you've met the Days Gone sound bug. The game's audio seems to suffer the most from some of the rough edges you'll find, and the noise cutting out 0- usually leaving your motorbike silent - is one of the most common issues. Although you might find weapons going quiet or even Deacon occasionally shouting when there's no need to.
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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.