Lost audio output for no apparent reason. No mute button. I know there is a fuse in the back of the unit...is it possible that this is for the amp portion of the unit?

Check whether the audio is muted by accidental press at remote control button. Enter its sound menu, and check it. If muted, as you know there will be no audio output. If not muted, check the speaker/s. If found OK; check and replace the damaged component/s at its audio output section circuit board. If needy, replace this board as card basis. If you wish to get some details; check the site linked here. Pull up older posts

Try this
1. Power off both the TV and the DVR.
2. Plug in the HDMI cable to both units.
3. With the DVR off, press and hold the power button on the unit until the word BOOT appears on the display.
4. Wait until the unit reboots and shows the clock again, then power the TV and the DVR up.
5. Choose the appropriate input on the TV for HDMI.
6. Use the SETTINGS button on the DVR remote to get into the "more settings" option.
7. Find audio settings and choose HDMI output, press select.
Hope that helps.........

Check whether the audio is muted by accidental press at remote control button. Enter its sound menu, and check it. If muted, as you know there will be no audio output. If not muted, check the speaker/s. If found OK; check and replace the damaged component/s at its audio output section circuit. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. Surf the site with patience.
http://electronicshelponline.blogspot.com/

Turning up the volume will usually cancel the mute too... sure it isn't an output issue or a fuse in the amp? If there is a button with ATT then this also might mess with the volume.

Check whether the audio is muted by accidental press at remote control button.Enter its sound menu, and check it.If muted, as you know there will be no audio output.If not muted, check the speaker/s.If found OK; check and replace the damaged component/s at its audio output section circuit board.If needy, replace this board as card basis.If you wish to get some details; check the site linked here. Surf the site with patience.Pull up older posts.Surf the site with patience.
http://electro-medical.blogspot.com/
http://electronicshelponline.blogspot.com/

It sounds that the audio output section circuit has the fault. Check it audio out pre-amp sockets at its back, connected to any external Amp. See whether audio comes out by this way. If not, the main digital board is faulty. If yes, audio output section is faulty.

It's muting out because you have a problem with one or both output transistors. Somehow they got shorted out. My speaker wires came loose and crossed blowing the outputs. I'm sure it's repairable but with an amp this old you never know what else burned up down the line. It was a great amp but because of its' age I thew it away.

Usually Laptop comes with sound mute button on the first row of your laptop or by the side.check out for that button.

If you can'r find them then

For Windows Vista & 7

Click windows button (Lower left),select "Control Panel ", Select "sound" Click "Comunication" tab>Make sure you select "Reduce the volume of other sound by 80%"

For windows XP & 2000 & 2003
Click Start button (Lower left),select "Control Panel ", Select "sound and audio devices" and click "volume Tab" and untick "Mute" 

Very well could be your amp setup. The only reason you would get lound sound from the stereo is if the mute is on or the audio outputs are fried. have you tried to hook up the stereo directly to a single speaker?

disconnect everything in the back of the unit. reconnect only the cd to the imputs. also,just to be sure, disconnect all speakers(label them). fire it up,play a CD, connect the speakers 1 at a time. If the 1st works you have hope ,if not, try each channel individualy.The one that reduces volume is the problem-maybe shorted cable, bad speaker,or , God forbid-Bad amp Channel. good luck