The 3.5mm jack on Android phones (beware, some are 2.5mm and will need an adapter) includes microphone input so you can make calls with hands-free headsets. So while you won't get stereo sound out of it, you can plug in 3.5mm audio from another source, and use one of the mic-to-speaker apps in the Play store to play that audio over the phone's speaker.
You will need an adapter to shift the audio signal to the appropriate contact surface. One of those Y-splitters which split the single 3.5mm jack into separate speaker + microphone jacks. Again, be mindful that some phones use a 2.5mm jack, and buy the correct adapter for your phone.
for some reason i totally forgot about the mic input being in there as well.
won't be the best sound but with an adapter you could get sound to come out of the phones speaker. personally i'd invest in a cheap speaker to plug the source into. for instance when i was deployed i bought a $10 set of pc speakers to take with me. we plugged in all kinds of stuff into it like laptops for movie night, mp3 players for music and even rigged them as an external speaker for the sincgars radio.
would be a much better sound than rigging up your phone.