From Audacity Manual
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The Repair effect can only be used on selection regions up to 128 samples long (for most audio, only a few thousandths of a second). Above this length, it becomes too hard to guess what should be going on in the section under repair. You will get an error message if you select too much audio to try and repair. In general, the shorter the section of audio you select to repair, the better the results will be.
The Repair effect is unusual because it requires there to be audio outside the selection region on at least one side of the section to be repaired. Examining this audio lets Audacity interpolate (make an informed guess about) the bad section. If the surrounding audio in the track is very short or non-existent there may not be enough information to make the interpolation, and an error message will be shown. For example, it is not possible to select a region up to 128 samples long in one track, paste or duplicate it into a new empty track and then repair it, because there will then be no audio either side of the region to examine.
- Select the approximate region of audio. Zoom in until you can start to see the individual dots (samples) in the waveform, and then reselect the shortest possible piece of audio for replacement.
- Click to replace the selected audio with the repaired audio.
- Zoom back out (or grab the handle on the horizontal scrollbar at the bottom of the screen and drag leftwards), click a few seconds before the region you were repairing, and play from there to hear if the repair sounds OK. If not, click and try a different region to repair. Include a little more on either side of the problem if the original damage was still obvious, or less if the repaired region sounded different, but incorrect.
- When trying to repair a section of audio that has been clipped, click first in the Track Control Panel to select all the track, then to make it quieter. This gives some headroom for Repair to work in. You can click to indicate clipped samples. If there is still clipped audio after Repair, , extend the region a little further either side of the clipping, then Repair again.
- If only one channel of a stereo track is damaged, you can repair only the relevant channel by clicking on the Track Drop-Down Menu and selecting "Split Stereo Track". After repairing the single channel, you can rejoin the channels by using "Make Stereo Track" in the same menu in the upper track.