The field I didn't know was already known and I either meant to come back around and finish it up, or just missed it here.
The field is the decoded length in bytes of all samples in the file. So, by getting the duration of the file in seconds and multiplying it by the sample rate and 4, you get the number of bytes. (multiplying by 4 since 32 bits per sample is 4 bytes per sample). I'm now figuring out what the bits per sample for the imported track is in case it's different than the usual 32 bits per sample.
The only thing I need to figure out is getting the number of channels from the mp3 library I'm using since it just has a string with channel names instead of just a number of channels. Might be overthinking that, but I don't really know much about music format so right now it's just assuming Stereo (2 channels).
I should probably test out .wav files again just to make sure I didn't break anything there, but then I'll upload the updated resource editor with fixed importing of mp3's
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