I readily found many "free" MP3 steam radiator hissing sounds. Even a youtube video for which there are several "free" converter programs - search "youtube to mp3 converter":
Many choices in sound modules. There are all-in-one modules where you press a record button and it has a built-in mic to record/save a 30 second (or whatever) sound. Then press the play button and it plays the sound over a built-in speaker. Simple and effective.
I was thinking of the ones that play MP3 files from a micro-SD memory card. For example:
In this case you apply DC power to the module and it automatically starts playing the MP3 sound file(s) on the memory card. A 128 MB card is about $1 on eBay and would hold hours of sounds - more than you'd every need for a layout sound application. Some modules can only drive earbuds so would need an external amplifier module (also about $1) but this has a built-in audio amplifier IC to drive a speaker.
I assumed you'd need to mess with an audio editing program such as Audacity. That is, you create your own layout sound by mixing the hissing sound with conversational clips. Yes, finding "clean" audio conversation clips (without background music or whatever) can be hard to find. I suggest making up some dialog and having some friends record it; hearing your own voice over and over can be un-nerving! There is a learning curve to using audio editors so if this is a one-off, then probably not the way to go.