To add to Alessandro's comments:
I think your files are too big for PlaySound.
If you're using C# or VB.NET I would use the built in AudioVideoPlayback
class. If your using C/C++ I would use DirectShow.
DirectShow can be a little daunting but for this simple application you
should have no problem. Just grab the current Platform SDK (Windows SDK),
and a recent DirectX SDK. Note if you are using Visual Studio 2005 you
want the Vista build of the Windows Platform SDK.
First build the baseclasses which are listed as samples, under
Load the project files from here to make this easy.
Read through the reference sample provided by Alessandro.
I also posted a full working sample project here for VS2005.
You need the following environment variables if you don't have all the
paths in your VS config.
DXSDK_DIR=<directX install location automatic>