In the manual, if you go to the section on the "External Control Device Setup", then scroll down to the "Command Reference List", Section 22. "Key", Command 1 is m, Command 2 is c, then you need the Set ID, which should be 1 out of the box, generally speaking, you don't have to worry about putting a 0 in front of it, ie 01
Then go to the list of IR Codes. Volume Up is 02, Volume Down is 03 and Mute is 09.
So Volume Up is mc 1 02\r
Volume Down is mc 1 03\r
Mute is mc 1 09\r

Therefore, direct channel commands are as follows,

0 mc 1 10\r
1 mc 1 11\r
2 mc 1 12\r
3 mc 1 13\r
4 mc 1 14\r
5 mc 1 15\r
6 mc 1 16\r
7 mc 1 17\r
8 mc 1 18\r
9 mc 1 19\r