The file size of an MP3 audio file depends primarily on how it's encoded and sampled. With Audacity, any MP3 file's encoding and sampling options can be adjusted to reduce its file size and make it easier to share and enjoy on your favorite devices.
LAME MP3 Encoder
Before you get started reducing the size of your MP3 files, download and install the LAME MP3 Encoder. While Audacity can be used to edit existing MP3s and export them to other formats out-of-the-box, it doesn't natively create MP3s. The LAME encoder adds the ability to save files as MP3, both when you change the properties of an existing MP3 and when you create a new project to save as MP3.
Reduce Sample Rate
Sample rate represents the number of samples carried per second in the audio stream, and effectively determines the range between the highest and lowest frequency sound in the file. Audacity sets projects with a default sampling rate of 44.1 kHz for optimal quality, but lower frequency settings can often reduce overall file size. The lower the bandwidth, the less high-frequency content is retained by the file, which may significantly affect your listening experience. To adjust sampling rate, click Edit from the menu bar, followed by Preferences. Select Quality from the list, then use the Default Sample Rate and Default Sample Format drop-downs to adjust sampling. Lower numbers in each will reduce file size, but also audio quality. For example, the 44.1 kHz setting produces CD quality audio, while 8 kHz sounds like audio over a telephone.
Convert Stereo to Mono
Converting stereo sound to mono doesn't directly affect the file size of an MP3 file, but it can improve the audio quality of the finished product. At the same bitrate, files encoded in mono almost always offer higher audio fidelity than files encoded in stereo because the available data doesn't have to be split between two tracks. To convert the track to mono, click the stereo track's drop-down menu, which is to the left of the blue wave-form graph. Select Split Stereo to Mono from the list to split the single track into two mono tracks. Click Tracks, followed by Mix and Render to combine the two tracks into a single mono track. Click the X* on the two original tracks to delete them, leaving only the single mixed mono track.
Bitrate determines how much data is allotted to each second of the file's play time. Higher bitrates usually equate to higher quality audio tracks, but also larger files. The highest setting available in Audacity is 320 kbps, while the lowest is 8 kbps. Depending on the original recording's quality and the type of speakers or headphones you're using, anything from 128 kbps should offer near CD-quality audio. Below 128 kbps, the chances of distortion and the loss of dynamic sound range increase -- at 8 kbps, the audio may be garbled beyond recognition. To change the file's bitrate, select File from the menu, followed by Export Audio. Select MP3 Files from the Save as Type drop-down, then click Options and select your desired bitrate from the Quality drop-down. Select OK and finally Save to begin encoding the file at the new bitrate.