When you’re putting audio tracks together, you’ll often notice that there are discrepancies between them. Using Audacity, you can easily change the tempo of a song independently of the pitch, or vice-versa, to better suit your project.
You can change the tempo of your track without it sounding like chittering chipmunks. Select your track (or a portion you want to edit) and go to Effect > Change Tempo…
You can enter a percent change, or enter the original and target beats-per-minute values. You can also optionally change the tempo so that it fits a specific length. This will lengthen or shorten your track accordingly, so you may have to rearrange tracks to make things line up properly if you’ve got a multi-track project in front of you. This comes in handy when you need to stretch out a vocal track to fit over some an instrumental part.
To change the pitch or key of a track, go to Effect > Change Pitch…
You can change the key via percent, frequency, the number of half-steps, or enter the original and target keys using the letter notes. This works really well for stringed instruments, like raising or lowering the key of guitar solo so you can play along in a different tuning.
Changing Both Simultaneously
Most people don’t want to change both at the same time, but it comes in handy when converting between Vinyl RPM formats. Go to Effect > Change Speed…
You can change between 33 1/3, 45, and 78 RPM formats or do a percentage change.
In general, you want to make small changes to your tracks so that they fit better. At larger changes, past 25%, you’ll hear noticeable artifacts pop up. This is especially true for vocals, and you can get away with a lot with some instruments. Used with high quality source audio, these changes can provide some really interesting effects, like making sci-fi noises and weird background rumbling.