Use audiomark to embed secret data into ".wav" files! Unfortunately only available to linux users, audiomark allows you to embed a string of data into a wav file via the "embed" command. You can then check to see if your friends got the same wav file

[DllImport( CoreDll.dll )] //, EntryPoint sound public static extern bool PlaySound(string pszSound, int hmod, int fdwSound) public const int SND_FILENAME 0x00020000 public const int SND_ASYNC 0x0001 public void sound(string soundPath