mitsu ws-55511 no sound please help

I bought a mitsu 55511 about a month ago, so far I love the tv except that four times when swithing from DTV(hd decoder) to my cable box or cable ant I seem to lose my volume. It will not come back unitll I go back to the DTV input and change the channel. Even if I shut the TV off and on it will not return volume untill I go back to DTV and change channels. Is this common with the built in decoder??