Bit Rate

• Bit rate refers to the amount of bits that are processed over time, usually measured in kilobits per second for audio files. Low speeds, such as 128 Kbps, lack the sonic clarity that a higher size, like 320 Kbps, can produce. Files that have been downloaded from file sharing and torrent services are often prone to a much lower quality than purchased music for this reason.
  
  

Format and Compression

• Compression is a post-production tool that is used to make the sound coming through a speaker system louder. Some file formats, such as AAC and MP3, are highly compressed and are smaller in size, which produces considerably less quality audio. Uncompressed formats, such as WAV and AIFF, take up a lot of space but sound much clearer. Formatting options can be customized in iTunes.
  
  

Speakers and Headphones

• If sound quality issues continue to occur with large file sizes, a last resort would be to verify that the headphones or speakers that are in use are in proper working order. Speakers that are even slightly "blown" can cause music to crack or to sound muffled. External wiring can also contribute to a weakened signal.