Google Nexus 5 — Nexus 5 Speaker doesn't work well‏

Hello,
I have Nexus 5 16gb mobile phone.
The OS is Android 6.0.1 (not rooted), security update from 05/August/2016 and build no. MOB30Y.
I didn't install any app lately and also didn't make any system updates from beginning of August.
There is a problem which appears yesterday that my mobile speaker work only sometime for few minutes.
I can't hear sounds or rings from the speaker. With regular conversation speaker and with headphones I can hear everything well. Until yesterday the phone was working fine, and suddenly the problem appears, it didn't fall down and didn't spill water on it.
In settings all the volume panels are up, but no speaker sounds.
I have made hard factory reset (master reset) to the mobile and wipe everything, and still didn't solved the problem.
IMPORTANT: I need to mention that from time to time there's suddenly sound, but after few minutes the sound disappears again.

For example, I logged in Safe Mode and check the sound, and it was ok. I could hear the ring sound and other. but after few minutes the ring and all other sounds don't work! I didn't open any app in that time.
Another example; yesterday from noon no sounds, and suddenly in the evening I got incoming call with ring sound (I thought everything is ok), and after that call all sounds don't work again.
I hope someone will offer solution.
Thanks.