Moto G Play gets wako. Speaker fault. How to find source of problem?

Every 24-48 hours my Moto G Play / Android 6.0.1 gets wako meaning: - (most often) During call I can't hear the other person and other person can't hear me, - (rare) Notification sounds are played very quietly (nearly silently) even though sounds volume is set to maximum and other tones (i.e. ringtones) are played correctly. Things that I'm sure: - restarting phone _always_ solves above problems, - if phone gets wako, problem always narrows to speaker / microphone; all other functionalities (SMS messaging, emails, Internet browsing, playing games, using apps) works normally, - these situations happens only on my phone; my wife has exactly the same model with exactly the same Android version and nearly exactly the same applications set and hasn't noticed this behavior so far. Things that I'm uncertain: - if I'm not mistaken described problems occurs only if phone isn't used very often -- i.e. if there was 6-8+ hours since last call made or received; if I'm calling often, phone doesn't get wako. How can I check or what can I do to check, if this is software (Android), network (LTE operator) or hardware (Lenovo) problem? What tests or operations should I perform? I could say (given that wife's phone has absolutely no problems) that this is certainly hardware problem. But then, restarting phone always solves problem, so this would indicate Android / software problem.