WavePad can perform basic editing operations (Cut, Copy, Paste, Delete, Amplify, Normalize) losslessly.
The MP3 format is inherently lossy: every time a file is encoded to MP3 some of its audio information is lost. The format is designed to minimize how audible this loss is, but if a file is repeatedly encoded and decoded then the cumulative loss can become audible. To work around this problem, WavePad can operate in 'Lossless MP3 Editing' mode, in which you can edit an MP3 file and then save it without re-encoding it. WavePad uses the original encoded audio, making changes to its structure and volume without decoding and re-encoding it.
Toggle Lossless MP3 Editing mode by clicking on the Lossless MP3 Editing button in the Home tab, or in the menu at Tools > Lossless MP3 Editing Editing mode. You can then perform editing operations as usual, but bear in mind the limitations listed below.
All editing in Lossless MP3 Editing mode must be made in chunks of 1152 (or sometimes 576) samples, or about 26ms at a typical sample rate of 44100. This is due to the nature of the MP3 format, in which MP3 frames always decode to this fixed number of samples. To make this easier, selections in Lossless MP3 Editing mode snap to 1152-sample boundaries. If you zoom in close enough then these boundaries become visible as dashed orange lines.
To save an MP3 losslessly, simply save the file to MP3 while in Lossless MP3 Editing mode. Any sections of unmodified audio will be saved without re-encoding them, but modified sections of audio will still go through the usual lossy encoding process. Audio which is cut-and-pasted can still be saved losslessly, except for a few of the frames on the boundary of a cut-and-paste, which may be re-encoded.
Some MP3 files start and end with frames which are shorter than 1152 samples. They achieve this by removing some audio from the start and end of the file. The audio removed from the start is called 'encoder delay', and the audio removed from the end is called 'encoder padding'. To allow for consistent lossless editing, when you enter Lossless MP3 Editing mode these removed samples are added back on to the start and end of the file. They will be removed again when you exit Lossless MP3 Editing mode.
When you use the Amplify and Normalize features with Lossless MP3 Editing switched on, they will work losslessly. They can do this by adjusting the 'global_gain' field of each MP3 frame, without needing to re-encode the audio data. Note that for this to work you must have Lossless MP3 Editing enabled before you use Amplify or Normalize. Also note that it is possible to cause clipping if you over-amplify the audio, or normalize it to a level above 100%. This is because in Lossless MP3 Editing mode the peak limiting normally used by and Normalize is not possible.