One of the main factors that influences an audio file’s size is bitrate. Put simply, bitrate is the amount of data processed over a given period of time. Files with higher bitrates sound better but require more memory. Audio with lower bitrates makes for smaller files, but sound quality is often sacrificed. The bitrate and filetype you need depends on the type of audio, what it’s used for, and who will be listening.
That being said, any alterations to the song file at all is going to being to reduce the overall quality of the audio file. If you can live with a supper high quality MP3, then you can go that route. Many people go back and remove any silence at the beginning and end of the song. This will not trim too much, but once they do this; many use 3rd party software and compress the audio file slightly to attempt to shrink their overall file size. Again keep in mind, that once you start manipulating the original audio file; you being to loose the quality that you started out with. So the million dollar question is " How far can you take the compression, before you audibly begin to hear quality loss?"
This can depend on what and where this audio file is going. If it is going to live on your phone or iPod/iPad; then you already are going to start looking at FLAC files and or various qualities of MP3.
