Convert and compress audio
Audio Tools has two tabs: Compress lowers the bitrate to make files smaller, and Convert changes the format. On Linux and Windows both need FFmpeg.
The tools read MP3, M4A, AAC, WAV, FLAC, OGG, OPUS, WMA and AIFF files. Video in the input, such as album art, is dropped.
Compress
Open Audio Tools. The Compress tab is open.
Add audio files.
Choose a format: MP3, M4A or OGG.
Set the Bitrate from 64 to 256 kbps, in steps of 32. The default is 128 kbps.
Select Compress audio (or Compress 4 files).
Results get _compressed added to the name and the extension of the chosen format, for example podcast_compressed.mp3.
Bitrate is how much data each second of sound uses. 64–96 kbps is enough for speech, 128 kbps suits most music on phones, and higher rates suit music on good speakers.
Convert
Open the Convert tab and add audio files.
Under Output format, choose MP3, M4A, WAV, FLAC or OGG.
For MP3, M4A and OGG, set the Bitrate from 64 to 320 kbps. The default is 192 kbps. WAV and FLAC are lossless and have no bitrate setting.
Select Convert audio.
The result keeps the original name with the new extension: voice.wav becomes voice.mp3.
Good to know
Converting an MP3 to WAV or FLAC does not bring back quality that the MP3 lost; it only makes the file bigger. Lossless formats help only when the source is lossless too.