Starting with Apple's iTunes 10, users are able to edit a specific song's playback length using the program's "Start" and "Stop" feature. Since this feature does not actually trim the length of the file selected, you will need to copy and convert the affected songs to iTunes' standard AAC format for the changes to take effect. You may then create a new mix or playlist and add the edited tracks as needed. The trimming and converting processes only copy and convert your tracks; they do not delete or replace the original files.
Editing Song Length in iTunes
Click the "Music" tab on the left side of iTunes to reveal your music library. Navigate to the first song you want to create a clip from. Right click its icon and select "Get Info."
Click the "Options" tab and press the "Start Time" and "Stop Time" boxes so check marks appear. Manually enter in minutes and seconds the "Start Time," depending on where you want your clip to start. For example, if you want the clip to begin fifteen seconds into the song, enter "0:15."
Enter your clip's "Stop Time" in the field provided, depending on where you want the clip to end. For example, to end your clip one-minute and fifteen seconds into the song, enter "1:15." Press "OK" to save your changes and exit back to your iTunes library.
Repeat the above steps for each song you wish to edit for length. Note that your edited songs' lengths will not yet appear in the iTunes library.
Converting Your Song Clips
Click the "Edit" tab at the top of the screen and select "Preferences." Press the "Import Settings" button on the right side of the pop-up window to enter the Import Settings window.
Click the "Import Using" drop-down box at the top of the window and select "AAC Encoder." Press "OK" twice to exit to your iTunes library and save your changes.
Right click one of your edited tracks and select "Create AAC Version" from the list of options. Wait for your clip to copy and convert to its new format. The song length will now reflect its previously edited length. Repeat the conversion process for each desired track.
Create a new playlist for your clips by clicking the "+" button in the bottom left corner of iTunes. Name your playlist and press "Enter" to save your changes.
Scroll through your iTunes library and drag and drop your edited AAC files into your new playlist's icon on the left side of the screen. Select multiple items by holding down the "Ctrl" key on your keyboard.