As there's been several topic about this recently I've made a plug-in for the task.
Because this plug-in is a Nyquist effect it is quite a lot slower than the built-in "Change Speed" effect, but some users may find it easier to use.
The plug-in has 2 modes of operation:
Mode 1: Stretch audio to a specified length.
To use this mode, "Select input Control:" must be set to "New Duration" (default).
Type the required duration into the "New Duration" box as two numbers - minutes and seconds. (the plug-in will also accept the duration as just one number which will be the number of seconds.
Example: to stretch the audio to 2 minutes and 25.4 seconds, enter:
Example 2: to stretch the audio to 200 seconds (3 minutes and 20 seconds), any of the following will work:
Mode 2: New speed as a percentage
This is different and hopefully easier than the "% change" used in the built in effect.
To use this mode "Select input Control:" must be set to "as Percent".
Use the "New Speed as Percent" slider (or type a number into the box next to the slider). The new speed is defined as a percentage of the original speed, so 100% is the same as the original speed.
To make the audio faster, the percentage must be increased:
200% is double the original speed.
300% is 3 times the original speed.
400% is 4 times the original speed.
Increasing the speed means that the audio plays faster (and higher pitch), so the duration will be shorter.
At 200% the audio will play twice as fast, so the duration will be 1/2 of the original.
At 300% the audio will play 3 times as fast, so the duration will be 1/3rd of the original.
At 400% the audio will play 4 times as fast, so the duration will be 1/4 of the original.
To make the audio slower, the percentage must be decreased:
50% is half the original speed.
25% is a quarter of the original speed.
10% is a tenth of the original speed.
The plug-in has two "stretching methods" - "High Quality" and "Low Quality".
"High Quality" is the default.
Both methods are quite slow, but the Low Quality conversion is not quite as slow as High Quality conversion.
When slowing down audio, there is very little difference in sound quality, both methods are very good.
When speeding up audio, the Low Quality method may produce "aliasing distortion".
Download the plug-in and put it in the Audacity plug-ins folder.
Restart Audacity to make it appear in the Effect menu.
The plug-in will be called "Change Speed (ny)..." and it will appear below the divider in the Effect menu.