How to best compress MP3/Flac so that download time is less?

Question

How to best compress MP3/Flac so that download time/data is less?
(like RAR and else)

I'm afraid I get restricted access from my ISP.. so I want to compress the files as much as possible... if the compression does make a difference.

Answer 1

MP3 are already highly compressed internally. There is nothing to gain there.
(You might be able to get them 1% or 2% smaller using RAR or 7zip with maximum compression settings, but that just isn't worth the effort.)

FLAC are raw audio without internal compression. Either RAR or 7zip these (but you will have to unpack them on the other end to make them usable again) or convert them to MP3 to get the smallest file-size.

Please note: MP3 is a lossy format. Converting a FLAC to MP3 will loose some audio-quality. (MP3 achieves its high compression by (among other things) throwing away some audio-quality.)
This loss is usually not noticeable unless you have very good ears and high-quality audio-equipment. Whether this loss is acceptable is up to you to decide.

Answer 2

As Tonny has noted, MP3s are already very compressed with an algorithm specifically designed for audio data. You will not get any significant compression using rar, zip, or anything else.

He or she is incorrect about FLAC files though - FLAC files are also compressed and you will not get any significant reduction in file size by re-compressing them either.

The only thing you can really do is transcode the FLAC files into MP3 files. I would use lame. Recommended switches for transcoding can be found here.

Note that re-transcoding an existing MP3 will result in much more distortion than transcoding a FLAC file to the same bitrate MP3. This is because MP3 is lossy compression, meaning that some information is discarded. FLAC on the other hand is lossless compression, so no information is lost.