15 month old waking up

hi my 15 month old girl has been waking up for a good hour to two over the past two weeks. she has slept through the night since about 11 weeks with an occasional few minutes wake up. now she wakes up as if wanting to play. shes getting harder to get bak off to sleep. i try putting soft music on to calm her which sometimes works but majority of times doesnt. shud i just go to a couple of times put her back down or just leave her to cry it out. my only concern at doing that is for my neighbours. whats causing her to wake up? sometimes she will have 2 naps a day, other times she only has one long one. wat shud i do? please give me some advice