Here are a few files you can try for a Windows based system:
Here is a sound effect link. I don’t know if you can put a voice over with it - Take a look.
Students can Insert a Soundtrack into their Compositions
It’s called Booktrack - it’s new in google and free.
You can apply sound and I think voice overs on your flyers. It’s Very Cool
Booktrack Classroom is a Web app that allows students and staff to easily add a rich soundscape to stories with dramatic effect. You can easily create your own Booktrack by matching stories you or your students create with music and ambient tracks from an extensive library of over 20,000 free-to-use audio files.