This can always be troublesome.
It is unusual to have interference in the audio but not the video. Are you listening through a stereo or theater system? If so and you don't hear it through the TV speakers the interference is likely there and not with the DirecTV or the TV set. Are you using line-in connections to the TV? In this case it's possible to have interference with audio and not video. Are you using a RF connection (channel 3 antenna input)? If so you should definitely be getting interference in both audio and video. In fact with mild interference it often bothers the video and not the audio because the TV picture is in AM, sound is in FM, CB is AM. The AM signals tend to mix easier while the FM section of the TV tends to ignore AM.
Older CBs were often more well made than newer ones (like nearly everything else). They often had better filtering. A radio can easily transmit signals it isn't supposed to if that isn't addressed (cheap radio). It's also possible the old radio has less power, due to age. Or perhaps the new one has been modified and makes much more power than it should. It could also be someone has "tweaked" the radio which is a common cause of trouble. When incorrectly adjusted the radio can make all sorts of signals it isn't supposed to.
What can be done? Well if he is indeed operating above the legal power limit you can get the law involved. Beyond that you have to determine exactly where this interference is occurring, inside the DirecTV box, into your connecting cables, inside the TV, into the stereo system, etc. You check this by swapping things around. Example, if you are listening through a stereo system, use the same cables and connect something else, a CD player, is the interference still there? If you are able to locate where the trouble is you can try using better connecting cables and/or applying snap-on chokes around them. These chokes are available at stores like Radio Shack. They clip around wires and are used to "choke" unwanted signals.
Make sure the CB antenna is properly tuned. Using good quality cable and having a properly tuned antenna (minimum SWR) helps alleviate this issue. With poor cable and/or poor tuning a lot of the signal can be "wasted" along the length of the cable, radiating from everywhere including inside the home instead of just from the antenna. Have a good ground. The CB set should have a good connection to Earth ground which insures the antenna system works as it should with the signal staying inside the cable. This is important for the best operation of the CB set too so it's in the roommate's best interest. Having a good ground at your TV can also be helpful.
