I have a hard drive full of mp3's at 320 kbps. Will I gain or lose anything (size and quality) if I encode them to a different format or bitrate? I like to get the files a bit smaller to add to my iPhone.
I don't think you exactly have a grasp on how codecs work.
There are two types of music files: lossy and lossless.
Lossless are files like : WAV, FLAC, ALAC (apple lossless codec).
Lossy files are files like : WMA, MP3
When you encode from lossless to lossy, you are ENCODING
When you encode from lossy to lossy, you are TRANSCODING *never do this*
When you encode from lossy to lossless, you're just wasting space
Encoding is great. When you rip from CDs, always rip via a lossless format. This means that EVERY single bit of the music will be saved as it was meant to be heard and as it was recorded. Use encoding to save space and transfer stuff to your ipod if you don't have space.
NEVER EVER EVER EVER transcode, that is to say, encode your 320kpbs mp3 files to another type of lossy mp3 file. Basically you will compress the file twice and it will lose a significant amount of detail, quality, and will result in a significant degradation in sound above 16,000hz and below 200hz. Most transcoded files almost completely delete all frequencies above 16k hz leaving you with something that sounds like a youtube rip. Let me know if you have any questions.