Windows Phone 7 doesn't support multiple MediaElement. In order to play multiple sounds in the same page, you may use the following approach.

    Add a reference to Microsoft.Xna.Framework

    Add the following using statements…

using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Audio; 

    Add the sound files and ensure that Copy to Output directory is set to Content

    Add the following method to your page…

public void PlaySound(string soundFile)
{
    using (var stream = TitleContainer.OpenStream(soundFile))
    {
        var effect = SoundEffect.FromStream(stream);
        FrameworkDispatcher.Update();
        effect.Play();
        //Thread.Sleep(effect.Duration);
    }
}

    You can now call this Method with a valid Sound file (WAV file) and it should play.
    Notice the commented code above. If the sound file take a while to play, you may decide to uncomment this line so that the entire sound clip is played. This would be a blocking call though.

Until next time,
Rahul