If you're a DIY enthusiast and don't mind working with low-cost electronic modules, you can cobble together an MP3 sound system more than suitable for layout sounds for less than $10.
There are dozens of MP3 player modules on eBay (free shipping) in various configurations - with speaker, without speaker, with memory, without memory (you add memory card), with amplifier, without amplifier (only drives earbuds), with power source (rechargeable battery), without power source (you supply DC power from battery or wall-wart).
I'd think the average person has enough stuff lying around whereby this can be done for, say, $5. Most guys have a spare speaker lying around, you probably have an obsolete cellphone/smartphone with spare microSD flash card - you only need the minimal sized card (e.g., 64 MB - NOT GB!) for all the layout sound length you can imagine. MP3 modules require DC voltage to operate - so if using AC Accessory voltage or track voltage you need to convert to DC.
Or as Shorling says, there are companies like ITT that put it all together:
If you're a DIY enthusiast, you can put together an arbitrarily unique layout sound using "free" audio editors like Audacity.
So you can piece together "free" downloadable MP3 trolley/transit sounds, station announcements ("next stop, Yankee Stadium!" or whatever you can imagine), city sounds, etc.. With MP3 player modules, even the smallest memory cards will have more capacity (hours or days of play time) than you can imagine. Don't constrain yourself by cheap audio modules limited to 10 sec or 30 sec of low-quality sound!