Hello! A friend of mine has an idea to do a sound installation involving a tree. The tree is in a park and there is no possible way to use a computer or any too visible (it may be stolen). The idea is that when a person approaches the tree an audio file, of a poem or song, plays. I thought of using an Arduino board with an MP3 player and an audio Bluetooth module that sends the audio to a speaker. for the sensor, I'm still undecided, perhaps a proximity sensor. I wanted to share the idea to see if it was possible and what other alternatives are available. Thank you all for your help! 
Hehe, talking trees. Do You intend to have a stand next by to sell pants?
You will Always face the need of battory power, charging etc. Adding,and hiding, solar panels looks difficult.
Funny idea...
I'm still undecided, perhaps a proximity sensor
Probably a "PIR sensor" or "motion detector".
What's the Bluetooth for? Why does it have to be wireless? Are you sending actual audio over Bluetooth or a "message" to play the audio? If you want to transmit the audio, it's probably best to get a "regular" Bluetooth transmitter (like you'd add to your TV, etc.) rather than something specialized for the Arduino or a microcontroller.
Railroader:
Hehe, talking trees. Do You intend to have a stand next by to sell pants?
You will Always face the need of battory power, charging etc. Adding,and hiding, solar panels looks difficult.
Funny idea...
Lol that's a nice idea to have a stand and sell pants. It never crossed my mind to use solar panels, but you're right, it seems difficult. I don't know what the possibilities are for power yet, that's why I was thinking on batteries, but they'll drain and need replacement.
DVDdoug:
Probably a "PIR sensor" or "motion detector".What's the Bluetooth for? Why does it have to be wireless? Are you sending actual audio over Bluetooth or a "message" to play the audio? If you want to transmit the audio, it's probably best to get a "regular" Bluetooth transmitter (like you'd add to your TV, etc.) rather than something specialized for the Arduino or a microcontroller.
It doesn't necessarily have to be wireless. I need a speaker that is small. Bluetooth speaker was the first thing that came to mind.
Use a dfPlayer Mini module. These are cheap and easy to use and take a micro-sd card with up to, at least, 99 tracks recorded on the card. The module can drive a speaker up to 3W, or if you need more power, can be connected to a small amplifier module and larger speaker.
PaulRB:
Use a dfPlayer Mini module. These are cheap and easy to use and take a micro-sd card with up to, at least, 99 tracks recorded on the card. The module can drive a speaker up to 3W, or if you need more power, can be connected to a small amplifier module and larger speaker.
Thank you so much for your suggestions!
I am imagining a crank generator with a return spring attached to the trunk and branch of tree so that as the branch sways back and forth, it pulls on the generator creating small amounts of energy. No clue how much energy something like can build up, but it would have the advantage of generating power incrementally as long as the tree is being pushed around.
I have seen camouflage game cameras that can’t be seen more than few feet away, should be pretty easy to do the same to your installation.