Ultrasound Time-of-Flight

Polaroid has used ultrasound sensors for auto-focus applications for many years. You can use the same type of sensors in micromice. Micromouse mazes are made from materials that reflect enough ultrasound for range finding purposes.

Ultrasound does have a few difficulties compared to IR. First of all, it takes a lot energy to produce enough ultrasound for ranging purposes. Your battery system must be capable of supplying bursts of high current. Ultrasound production is difficult to focuse, which makes the out going cone kind of large. This means it is difficult to tell what reflected the detected ultrasound.

Ultrasound sensors are not cheap, either. They are typically more expensive than IR triangulation type sensors.