Sound volume on Samsung Omnia II

Question

I just had a customer report to me, that when our app plays a sound on the Samsung Omnia II, it is played not as loud as when the system itself is playing it.

The customer uses the very same sound for our app as for receiving SMS, and when receiving a SMS, the sound is played much louder than when our app is playing it.

I'm using SndPlaySync to play the sound

[DllImport("aygshell.dll", SetLastError = true)]
static extern IntPtr SndPlaySync(string Path, uint Flags);

Why does this not work on the Samsung Omnia II as on other phones? Is there a workaround for this?