Your audio collection does not fit in the memory of portable music player. Sound familiar? There are several ways to solve this problem: change the audio codec, compression options or remove redundant information from the audio file. To do this, you will need audio converter and tag editor.
The simplest and most effective way to reduce the size of the audio file is to use lossy compression algorithm. Lossless codecs like FLAC or APE can reduce the file size of only two times, but lossy codecs like MP3 or OGG reduce it by 10 times or more. The most efficient lossy codec is MPEG4. If you have a collection of audio in MP3 format you can convert it to MP4 and reduce the size of up to 30% without loss of quality.
Another case is when you already have a track in a lossy audio format and want to make it even smaller. Easily! You can just reduce the bitrate of the audio file. At the same time the sound quality will be reduced proportionately. Most people can not notice the difference between 320kbps and 128kbps on a portable player, but the file size reduced by half.
Lossless formats do not allow you to set bitrate, so the only way to reduce the size of the audio file is to reduce the sampling rate or bit depth. HD audio format provide great quality for Hi-End Audio Equipment, but it's redundant for portable systems. If you have HD audio track with 24 bit depth and 192kHz sampling rate then would be quite appropriate to have a copy of the standard quality 16 bit/44100Hz. For Podcasts and audio books you may reduce the sampling rate even to 22050Hz instead of the standard 44100Hz. Additionally, you can convert stereo to mono.
You have to optimize compression settings, but you are missing quite a bit of space? It's not so bad! Most audio files contain meta data that can be removed. The most common ID3 tags inside MP3 files may take tens of kilobytes.