I wanted to add some ambient sounds to Die Keinnamebahn. Not wanting to have a wired in solution I found on eBay some inexpensive (<£5) self-contained mp3 players with 2 inch speakers. The units have a 4 hour rechargeable battery and can use mp3 sound files from either micro SD cards or USB thumb drives.
I bought three of these devices and produced suitable audio files for the ferry area, station and farmland areas.
I used free downloadable sound files which I combined and edited using the free Audacity audio editing software and Lame mp3 plugin.
Here is the prototype mp3 audio file I produced for the ferry dock area which has ships engine, horn and seagull sounds.