You can control this box this pretty cheaply using a Comcast cable remote.  Some other cable company remotes will work as well.   eBay has lots of these, and you can usually get a new remote shipped for $6. Probably you will need to do some manual programming to get the functions you need.   

First, use setup code 31308 (or AUX 1308 if the remote has a grey OK/Select button). That should control volume and other mainstream controls.  To switch from stereo to surround, you'll probably need to do some experiments.  Unfortunately, Panasonic HT systems don't all use the identical IR signals for these sorts of functions.

Choose a button on the Comcast to use for stereo/surround.  You can program this individual button to send any signal used by Panasonic HT systems using the 994 command.  There's a good chance that EFCs 60726 or 44087 will work.

However, we don't know for sure the correct EFC for your HT system.  If the above EFCs don't work, use the Panasonic Audio Master spreadsheet to identify other possible subdevice and OBC combinations.   You'll also need RemoteMaster.  Load this SA-HT15 upgrade, and modify the subdevice/OBC columns to values suggested in the spreadsheet.  Try the corresponding 5 digit EFCs which RM calculated.