Linking together ‘iPhone 7” and “audio” in the same sentence is a red flag for the classic problem—-iPhone 7/7+ audio ic Master Clock line failure. This is a very common fault and it shares the same underlying biology as the now epidemic iPhone 6+ touch disease.
In the 7/7+ the normal use of the flexy/bendy iPhone pulls and tugs on the logic board since it is screwed down into the housing (as opposed to similar large phones from other manufacturers where the board floats in the housing). Over time, that flexion will cause a weakening/lifting/separation of the C12 pad under the audio ic that controls the master clock function of that chip. The result—- a wide array of individual symptoms of audio-related faults such as:
iPhone 7/7+ audio ic master clock fault symptom list (can be any or all):
Grayed out voice memos
Callers can’t hear you on calls
Siri can’t hear you
Headphones not working
Long boot time (3-5 minutes)
Hangs at Apple logo
Laggy touch when it does boot (with older iOS versions)
Of course you should rule out parts first—-try a new dock and known good headphones. But if the problem persists, then head to your local microsolderer or mail in repair service for the iPhone 7 audio ic solution—-running a microjumper at the C12 pad to strengthen it, then reinstalling your audio ic on top of the jumper.