The "punch in. punch out" concept goes back to the days of tape based recorders - if you have only 4, 8 or 16 tracks available then "punching in" and overwriting a previously recorded track is often the only practical option. However with Audacity and modern computer hardware, the number of available tracks is enormous, so there is no longer the need to be frugal with track (or tape) usage.
This is copied and pasted from a previous post so the shortcuts will not be quite right for a Mac.
When it says to press the "Ctr" key, it is usually the Command key on a Mac.
Doing a simple "punch in / punch out" with Audacity 1.3 (note that if you are not familiar with the shortcuts (hot keys) all the commands are also available from the menus):
- Record track 1
- Let's say we've messed up a section approximately 1 minute into the track.
- We find a sweet point just before the mess to "punch in" and a sweet point just after to do the "punch out".
- Click on the track info box to select the track.
- Click & drag from the punch in point to the punch out point
- "Ctrl B" to label the region - type "bad bit" as a name for the label, then "ENTER" to close the label and "ENTER" again to deselect the label track.
- "Ctrl Alt K" to "split delete" the offending bit
- Click a few seconds before the punch in point
- "R" to record - play and re-record the new bit on audio track 2 - press "stop" at the end.
- If you mess up again, "Ctrl Z" to undo and try again.
- When you have got it right, click on audio track 2 to select it,
- Click on the name of the label ("bad bit") to select the labelled region of the new recording.
- "Shift Click" on the track info for the label track to deselect it
- Go up to the right hand edge of the selected region of audio track 2 and drag the selected region to the left so as to include all of the recorded section prior to our "punch in" region and "Ctrl Alt K" to "split delete" it.
- Repeat this method to delete the trailing region of the new track after out "punch out" point.
You now have a perfect "punch in / punch out".
This may seem like a lot of steps, but I've put in all the details - once you are familiar with the short cuts it's as simple as
1) mark it
2) delete it
3) re-record it
4) trim it.
When using labels to mark the positions, you can also use cross fades between the old and new recordings - brilliant if there are no good punch in / out points (and something that is just not possible with traditional punch in punch out),.