This should not be necessary. You can change the output volume using the mythtv setup, from the mythtv frontend (normal viewing mode)
To fix this do the following:
1. Fire up MythTV from the "TV" menu selection just like normal.
2. Once MythTV is up and displaying the show, either using an MCE remote or the keyboard invoke the LMCE menu.
Keyboard use the F7 key - From the menu on the far left, select "Recordings"
MCE remote, press Recorded TV (same as selecting recordings from the left most LMCE menu)
3. Escape back one MythTV Menu Level
Keyboard use the ESC key
MCE remote use the back arrow
4. Go down and select the "Utilities/Setup" option - Hit Enter
5. Go into the "Setup" option (Should be the only one available) - Hit Enter
6. Go down into the "TV Settings" menu - Hit Enter
7. Go down to "Recording Profiles" menu - Hit Enter
8. Select "Transcoders" - Hit Enter
9. Select "LiveTV" - Hit Enter
10. Navigate through there until you get to about the third screen go to where it says "Volume"
Use the Up/Down arrow keys on the keyboard
Use the directional pad Up/Down on the MCE remote
11. Change it to the desired volume
Use the Left/Right arrow keys on the keyboard
Use Left/Right directional pad on the MCE remote
12. After you have it where you want it, arrow down with keys or directional pad to "Finish" and hit enter.
That should be all you need to do. Play with the settings until you are comfortable with the levels. These should stick and remain that way.
I hope this helps. Once you are done with all your settings, and have exited out to the main menu (ESC or Back Arrow), Go back to Live TV
Keyboard use F7, and then select "live" from the left most menu.
MCE remote, just press "Live TV"