wvmarle:
Only if you can make sure somehow that this error is no more than say 10 µs (about 3 cm in distance) you can use this to time how much longer it takes fro the sound to arrive and have a reasonably accurate measurement.
A round-trip nRF24 message (message plus acknowledgement) takes a few millisecs. But I have never had any reason to check how consistent the round-trip time is.
However it could vary significantly if there was wireless interference causing it to make a number of retries before it was successful. That's why I thought that basing the timing off the receipt of the acknowledgement might be better. However I had not done the maths and had not realized that 10 µs accuracy would be needed.
A simple light beacon might be a more reliable means to start the timing.
...R