One of the coolest things about a trip to Africa is hearing the night sounds. Nothing prepares you for the soundtrack that is nearly constant. So many birds, insects and animals are around, day and night, that there are only a few hours in the early morning, around 3 or 4 am until 5 or 6 am, when things get quiet. The frogs have worn themselves out by then and even the night birds have gone to bed. I highly recommend camping for the best chance to hear animals, as lodges have music, drunk tourists, and other distractions.
Among the sounds are crickets, cicada, frogs, birds and, if you're lucky, mammals. Lions, hyena, leopard, zebra, elephant.....there are all sorts of noises you might hear.
If you want a taste of these sounds, here are some neat links to check out:
What I call "tinkle frogs": http://amphibiaweb.org/sounds/Amnirana_galamensis.mp3
What I call "bubble frogs": http://amphibiaweb.org/sounds/Hyperolius_nasutus.mp3
Mammal sounds: http://www.africantrophy.com/sounds.html
More mammals: http://www.partnersinrhyme.com/soundfx/animals/Safari.shtml
Elephant sounds: http://www.elephantcountryweb.com/Elephantsounds.html