The device no longer changes channels either manually or by voice cammand. Changed batteries but still no ability to change channels.
Sorry to hear you are having a problem with your Comcast remote control. I would like to suggest a few things before having you exchange the remote control.
First, try changing the batteries in the remote control (Sounds like you already tried this)
Another thing to try would be to unplug your cable box from the AC power outlet and then plug it back in again. That may straighten things out.
Sometimes there can also be interference with compact florescent light bulbs (CFLs) in the room (details below).
If you are still having a problem, you can exchange your Comcast remote control at a nearby service center (link below) at no charge or call us at 1-800-
COMCAST and we can arrange to get a replacement remote control or box out to you.
Compact fluorescent lights and TV backlighting have both been known to interfere with commands from a TV remote control. There could be interference with CFL (compact fluorescent bulbs) or any IR (infrared light) coming from the actual TV set. If you have CFLs in the room, try turning them off and then trying the remote control to narrow down the issue.
Also, turn off the TV set and then use the remote on the cable box to see if the cable box is more responsive (watch the LED front panel display). That may indicate a setting with the TV backlighting that may need to be adjusted.
From wikipedia below: http://en.wikipedia.org/wiki/Compact_fluorescent_lamp
Infra-red signals Electronic devices operated by infra-red remote control can interpret the infra-red light emitted by CFLs as a signal, this limits the use of CFLs near televisions, radios, remote controls, or mobile phones. 
A good resource on this topic here: http://forums.x10.com/index.php?topic=16475.0