Another tool caught my eye when I was looking at TEncoder. This is audio only, but it can extract audio from video files and either copy it or convert to other formats. It can take audio files directly as input of course.
The name is TAudioConverter and it is obviously made by the author of TEncoder. Even the GUI is quite similar.
TAudioConverter has some interesting quirks in its layout. For one thing, you don't choose an output by file type. Rather, you choose the encoder that you want to use. So if you want an AC3 output, it is assumed that you know that Aften is the encoder that you need. TAudioConverter has a couple of good AAC encoders QAAC and FHG AAC, so there is no need to use FAAC that is also included. Lame is available for mp3 output. And there are quite a few other options, including Ogg Vorbis, FLAC, Opus, Monkey's Audio, WAV, etc.
My tests included straight conversion from mp4 audio files to mp3, which went very well and quickly. I also tried extraction from vob files into mp3, which was again successful.
One of the last tests I tried was extracting the AAC audio from an mp4 video file and converting to AC3. This last test converted properly and plays correctly but the tag showing duration of the file is incorrect. The original audio is 42 minutes, but the AC3 file shows to be over 3 hrs. But as I said, it plays correctly and isn't actually any longer than the original.
One common task in audio conversions is changing the sampling rate. CD audio is 44100, but audio found in dvds is 48000khz. To change this setting, go into Effects Filters, put a check mark in the box to enable this function then you can change sampling rate. There are some other options there as well, such as Normalization and changing channels.
All in all, a handy little audio converter.