Some documentation from the source (platform/generic/wrappers.c)
void play_sound(unsigned sound)
{
static const int sounds[]={ 0x2001, //startup sound
0x2002, //shutter sound
0x2003, //button press sound
0x2004, //self-timer sound
0xC211, //short beep
50000, // AF confirmation
0xC507, // error beep imo
0x400D, // LONG ERROR BEEP CONTINIUOUS- warning, cannot be stopped (yet)
};
if(sound >= sizeof(sounds)/sizeof(sounds[0]))
return;
_PT_PlaySound(sounds[sound], 0);
}
7 happens to be the "long error beep"