FREE Baby Sleep Instant App for Android @ Google Play
This free app for Android, Baby Sleep Instant, is the most popular free app of this kind, and the reviews are really great. It can play all sorts of 'white noise' of the sort that can soothe and calm a baby (and an adult!) and help them settle to sleep. There are sounds of a heartbeat, car, train, vacuum cleaner, running water and lots more like that.
Apparently these types of repetitive constant 'white noise' sounds have been shown to be more effective than songs, lullabies or music in getting babies to fall asleep. That said you can also use this to record a lullaby or your voice to play back to your baby as well.
The Baby Sleep Instant App is designed so that you can choose your sound, set the times and place your device in the baby's room so that they can hear the sounds. You need to set your phone to aeroplane mode though, or they might be awoken by text alerts or ring tones!
There are other free apps like this available like this for Android, and if your baby is having trouble settling themselves to sleep then you've nothing to lose by trying it. Out of more than 16000 reviews, 12000 are 5 stars - worth a try?