If you inserted your audio "snippets" from your media browser, you should be able to set start and finish points.
With the audio snippet selected: Inspector > Quicktime > slide start and finsih pointers where needed
Note: using this method the accuracy of the settings may not be exactly to your desired points and there are no fade-in or fade-out controls so some fiddling may be required.
It would be better if you could edit the audio files in an audio editor such as Garageband and save them as seperate files and place them into your presentation. That would allow you to retain the edited length and any fades.
Better to use an audio editor to create a compilation track with X fades and then drag it in from iTunes to the Soundtrack well in the Audio window of the Documents inspector.
Adjust slide timing to suit the audio.
You can use the freeware Audacity...
... to edit the audio and create the sound track...
... and export it to MP3...