you are very laconic in your description, and the answer can only be very broad... the issue can be anything, from loose connectors, to water-damaged modules...
to narrow down the problem, you need to try several things and make a note of the results... next time you lose your sound, try to see if it correlates to a bump on a road, to high temperature, to rain, humidity... note if the nav is going blank, or frezzes or whatever... try to bypass the nav module and see if sound comes back...

we are trying to help you diagnose the issue but we are remote to you and need first hand account in order to help... and as much info as possible...