Since the audio is already in AC3 format, can I still restore that audio, or do I have to just keep it how it is?
AC3 is just an audio format. Sure, it can be improved. Any audio can. The only issue is that AC3 is compressed, so it's often nor workable directly in editors. Like video, audio editors prefer to work with lossless or uncompressed audio files, such as WAV. So you'll want to convert the AC3 to WAV, then work on it from there.
I convert AC3 to WAV using Goldwave, because I have an AC3 codec installed in Windows XP. It's quicker than most other software. The freeware Aften converts to AC3, but I don't remember if it converts AC3 to WAV. It might. Same for BeLight. Avidemux
can probably convert AC3 to WAV. I don't think Audacity opens AC3 -- it won't on my computers.
I edit in Sound Forge
. Sound Forge
9 can open AC3 directly, though I still prefer to give Sound Forge uncompressed WAV files because the program works quicker without the decoding processes.