they're using AAC for the audio which has many benefits compared to mp3 or wav. MP4 also offers more comprehensive metadata options.
> Some radio broadcasts were stored as mp3 and wav files, with accompanying checksum files. Other broadcasts were only stored as mp3. Before the re-archiving process began, it was decided to generate new MP4 playback files from the wav files to replace the varying qualities of the old mp3 files.
In the cases where we only had mp3 files, the mp3 file was preserved as our master in the preservation environment, with a copy sent to the streaming servers.