As far as I know you can't "Stretch" audio layers as you can video layers. You can often do that in an audio program (Audition), but you may or may not get the result you want. If you merely want them repeated just add them to the timeline again
You can make them longer the same way you make video files longer. You can mess with the Time Stretch properties or you can enable Time Remapping.
That being said, messing with the time of audio will also mess with the pitch. So, if you don't want that to happen, you'll want to use an audio app to mess with it (like Audition) where you can stretch a clip out, but maintain its pitch. Or, as Brian has suggested, just lay out a second copy of it.