I'm not sure I've understood properly, but it seems to me that Sony
Vegas is not a DAW, but a video editing software...
It is plenty of audio editing and mixing software that can open MP3 or
AAC (AC3) files, but what they do is they all convert the compressed
files in an uncompressed PCM format, and then perform all the mixing and
editing operation, and convert them back to the compressed format after
the processing. This is fore sure a good method with short files, but
having for example a ten ours MP3 stereo recording, on a normal PCM
format would be nearly 10 Gb of data, therefore for mixing and editing
directly in MP3, in this case, could be a very good idea.
I'm not aware of any available software that directly mixes compressed
files, but for the editing you could use http://mpesch3.de1.cc/ , which
allows you to cut MP3 files and change the volume...
A friend of mine has been working on a software for the mixing directly
in compressed format (Dr. Giancarlo Vercellesi, you can find online some
of his publications), and there are some software prototypes for this,
but nothing available yet...
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