If your TV and sound system both have a Digital Optical Out/In you can use a TOSLink cable (an optical cable) to connect your TV directly to your sound system, and remove the connection between the DISH receiver and the sound system. As long as your sound system is on the right input, all sound accompanying the picture on screen will go to your sound system. Most TVs also have an option to turn off the internal speakers, if not just reduce your volume all the way down to avoid that weird ghost sound effect. If you use your sound system all the time for volume, you can program the sound system to your DISH remote's AUX, and even set the remote so that it only sends volume and mute commands to the sound system (that way you don't have to dance between sat, tv, and aux all day, just leave it in sat mode).
To program your remote to the sound system, follow the instructions on the sheet you got with the remote (yeah, right like anyone kept that) http://www.mydish.com/support/remotecodes
you can use that link to program your remote. You can also use the learning mode if your remote has a little glossy square on the top left side and you have the original remote to the sound system. (Learning mode can be gleaned from the remote manual Remote Control Manuals
After programming the aux device, and verifying that it changes the volume up, down, and mute, THEN:
Hold AUX until light blinks 3 times, hit pound (#), Volume Up, 0 (Zero), pound (#), SAT, then test to make sure it goes up and down.