There was a good article on layout background sound from c.d. players in the January, 2012 issue of Model Railroader.
For your needs, the Model Rectifier Symphony 77 is what you need -- sound under the layout without DCC.
http://www.modelrec.com/search/product-view.asp?ID=7614
I have a small N scale layout representing the transition period between steam and diesel. I would like to add sound but don't want to have the expense of DCC. I prefer manual operation anyway.