Are you using any third party apps for your Audio, I use the EQ app on my phone on your M if you use EQ you'll have to disable the internal EQ that comes with the phone.. so there is no interference..
rbtnt Let's help get your volume working. Have you tried adjusting the volume in settings for notifications?
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How about trying any Resets like power reset.! Clearing of the Cache for the individual app that is causing the issue or you might try doing a system cache clear..