It's worth giving the 44,100Hz, 192kbps, CBR rate a try. Unless you are one who has carried out a lot of comparisons between MP3 and CD audio at various bitrates on high end audio equipment (or audiophile headphones) over a long period, you'll unlikely notice any difference between 192kbps and 320kbps. Most people I know are even perfectly happy encoding their music at 128kbps.
The best way to find out which bitrate you're comfortable with is to start at a low bitrate such as 96kbps, so you hear what defects MP3 introduces when encoded at too low a bitrate. Then try another encoding at 32kbps higher each time until you can no longer distinguish between the encoding and the original file. Finally, to be on the safe side, add another 32kbps to this and there's a good chance you'll not find any song you can distinguish between the two.
Finally, think of who'll be watching the videos. E.g. if the music is just there to add background music to the video, chances are that no one is going to notice any minor defects in the music, if they're paying their attention to the video. To give an example, YouTube encodes their videos with 64kbps MP3 and even their HD videos are only given 128kbps.
The difference of 32kbps is 14MB per hour, so if let's say you decided to reduce your audio bitrate from 320kbps to 160kbps, this would be roughly a 70MB reduction in the encoding size for every hour of recording.