How can sound travel so fast in the interstellar medium?
The catch is that you can only transmit very low frequency sounds - ti can be thought of like the variations in travel time for any individual particle drown out any high frequency signal.
It's hot, so the speed of sound is high.
Given the impedance mismatch[0], even the parts of the solar system outside Kármán lines where the interplanetary medium can support pressure levels equivalent to normal speaking (including low Earth orbit), I'm told human ears can't respond to that pressure change properly.
Sensors can be built to pick it up, but that may not be in scope for your question as we can also do that for acoustic waves in the CMB.
[0] https://en.wikipedia.org/wiki/Impedance_matching#Acoustics
Not sure if it would actually be useful, because I'm sure radio waves would be much more practical, but sound in space is a fascinating idea.