If you need to edit your sound files, there are several free audio editing programs available. Many of these are great products that work well. You can find them by surfing on the Internet.
Many computers come with program packages that include tools for simple audio editing.
If possible, plan your voice-over well ahead and develop a script. The secret of creating a good flow in a prezi is to match the pace of the audio with the visual information. Set a pace that allows the viewer to see and absorb all of the information, without allowing so much time that the presentation becomes boring.