First try restarting the phone (turn it off and back on) to see if that resolves it. If not, then check any apps you may have recently installed that could have altered the sounds on the phone. If it still happens, then try starting up the phone in "Safe Mode" and see if it happens in there. You will have limited access in that mode, but if it doesn't happen in there, then you know there is someone on the phone causing it. Bad app, bad update, virus, malware, etc. that will need to be removed to make it work correctly.
Should none of this help, then I would have it looked at, as it is probably a hardware issue.
To get into "Safe Mode" please do the following.
1. First turn off the phone.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable it, simply repeat the steps above, and you should see "Safe Mode: OFF" rather than on.