Dinnerbone did this, though through a command, not a new block.
/playsound <sound> <player> [x] [y] [z] [volume] [pitch] [minimumVolume]Is the main way it is set up.
"playsound" is the command used, for the command block, or console.
The <sound> part is how it emits what sound, and goes through the game's code-names for the sounds it has to play at certain parts.
/playsound ambient.weather.thunderThis would play the in-game Thunder sound effect.
<player> determines who hears it, so in a map only certain people can hear it.
/playsound ambient.weather.thunder @a[team=Red]This would make only the people on the Red team of a Mini-Game map would hear thunder.
[x] [y] [z] Helps check where the center of the noise is, like how the center of a Pig-Oinking sound effect is the Pig.
/playsound ambient.weather.thunder @a[team=red] 1642 85 -625This would make the center of the Red team only thunder noise come from those coords.
[volume] checks how loud the noise is, or how quite. The volume can be set from 0.0-2.0.
/playsound ambient.weather.thunder @a[team=red] 1642 85 -625 1.6This would make it quite a bit more than the medium amount of noise that can be emitted.
[pitch] is how high-pitched or low-pitched it is. Can be from 0.0-2.0.
/playsound ambient.weather.thunder @a[team=red] 1642 85 -625 1.6 1.9This would make a very high-pitched noise, which could sound almost like an alarm noise, in some sort of Race style mini-map.
[minimumVolume] is how low the volume can be, in the case it having randomized volume.
/playsound ambient.weather.thunder @a[team=red] 1642 85 -625 1.6 1.9 0.2This would make the thunder noise sound farther away, in the case of one command block as a replica for actual thunder.
This is all the perimeters for the command, to create an alarm noise for the Red Team.
The finished product:
/playsound ambient.weather.thunder @a[team=red] 1642 85 -625 1.6 1.9 0.2
With the new "Resource Packs" able to change sounds, creating a map with actual sounds, and an actual Narrator is quite easy. Hope this helped!
Also, if you don't know the sound effect names, then here is how to figure out.
go into .minecraft (or minecraft in iMac), and go into the "assets" folder.
go into the "sound" folder, and you should see a bunch of folders. I am going into "mob".
then you should see a list of folders, each one should be named after the mob, I am going into the "creeper" folder. Then, you will see all the .ogg files, named after the effect, and if you went into certain folders, such as "fireworks", you will see the .ogg files straight away, instead of more folders, since I went into creeper, I saw the "death" sound effect.
So, you will figure out by doing this, and in the end, the sound effect I figured out is "mob.creeper.death", or if you went in to something like firework, then "fireworks.twinkle1", since there is only one folder. As well as for a name that has a "." in it, then you right a "_" instead, "random.anvil_use".
Experimenting results in strange things. The community, and I have discovered things that may have not been discovered until this was released.