What's best is having three or more tranceivers (both TX and RX capable) What's even better is also having an RF link between them all. An RF pulse travels at very close to the speed of light. Sound doesn't. If you send the RF pulse at the same time as the audio pulse and have the receivers time the difference in pulse reception You have a differential measurement of the distance between the RF transmitter and audio transmitter. It's basically using the RF pulse as a master clock synchronizer. If you add a two way communication link all the devices can discuss amongst each other their location. Although designing the protocol would be a little difficult.
Have you considered sending a IR flash and ultrasonic signal at the same time? For the distances involved IR light travels so fast you can assume it's instant. You can use the receiving of the IR pulse to basically tell you when the ultrasonic pulse was sent (since we are assuming IR light is instantly fast). THat way you can just time the length between receiving the IR light was received (basically when the ultrasonic pulse was sent) and time how long it takes for the ultrasonic pulse to arrive. YOu can use this with the speed of sound to figure out how far away the thing is.
You can do this with 3 stationary receivers and 1 mobile transmitter. The transmitter sends both IR and ultrasonic. The receiver senses both IR and ultrasonic. You can also use RF isntead of IR (basically the same thing, different wavelength of radiation). If you use this method and onyl have 1 transmitter, you don't need to differentiate between different ultrasonic/IR pulses.
EDIT: I realize that I said the same thing Sceadwian did.
If you use one transmitter and two receivers with known distance between them you can easily make triangulation,But if you use one receiver and two transmitter you have to use a method called "double pulse separation method" in order to distinguish which transmitter send the pulse .more information about this method could be found in the following paper