Are you using FF / REW on an internet channel recording? If so, that is a known bug and you need to use Skip instead.
I sometimes get loss of sound when changing channels, can't remember if it is from a terrestrial to IPTV chan. or vice versa. My normal fix is to toggle the HDMI output from 1080p to 1080i or 720p then revert changes when asked. Very occasionally do I have to reboot the box.