OK, I'll assume that you have a TX-SR607. Volume lock means that the TV volume will be adjusted when the remote is in cable mode. On the Comcast remotes that I have (red Select/OK button), contrary to the instruction sheet, volume lock does not apply to the AUX button, and it always sends its own volume code. If you want to make the AUX device control volume while you are in cable mode, press setup, two blinks, 993, two blinks, AUX, two blinks.
You can use the Comcast remote to control most of the receiver functions, provided that you are willing to spend a few minutes typing in some numbers. First you need to set the AUX mode on the Comcast to 30135. That specifies NEC1 protocol, device 210, subdevice 109. That should control receiver volume. It should also turn on the receiver, but not turn it off. Perhaps some other buttons will work. For those that don't, you can use a keymove to get the functionality you want. In total, the Comcast remote is limited to about 70 keymoves, which should be enough for the functions you need here, and for the TV, if any are needed there. Speaking of the TV, why not tell us what you have? Maybe we can help.
In Aux mode (using setup code 30135), press setup, two blinks, 994, press setup briefly, any of the below 5 digit EFCs, and then the key you want the fucntion assigned to. Should get two blinks. If you want the digit keys to work, I can post how to add those.
Code:
ON/STANDBY 27493
DISCRETE ON 60517
DISCRETE OFF 57957
CH/DISC + 60773
CH/DISC - 61797
VOL UP 61285
VOL DOWN 62309
MUTE 61541
GUIDE/TOP MENU 30053
SETUP 41829
RETURN 40037
AUDIO 42085
ENTER 47717
UP 63333
DOWN 64357
LEFT 62565
RIGHT 63589
CLR/SLEEP 08293
STEREO MODE 23653
DVD/BD 29797
VCR/DVR 29285
CBL/SAT 28261
GAME 28773
AUX 14949
TV/TAPE 28005
TUNER 29541
CD 29029
REW 12133
PLAY 45157
FWD 12645
PAUSE 44645
F.REW 47973
STOP 45925
F.FWD 46181
DIMMER/+10 47205
DISPLAY 41061