It just means it works or operates "by means of" sound, i.e. using sound.
Sonar is een techniek waarmee je voorwerpen kunt ontdekken door middel van geluid (onder water).
Sonar is a technique with which you can detect objects (under water) using sound.
Mijn batterij is waarschijnlijk op en nu kan ik hem door middel van geluid niet meer vinden.
My battery is probably run down and now I can no longer find it using sound, i.e., I won't be able to find [my cell phone] by listening (for the ring tone) because the battery is probably run down. ('hem' is most likely referring to a cell phone.)
Kan je helpen een vogel te identificeren door middel van geluid?
Can you help identify a bird using sound? i.e., by listening (to its call)?
So, as you can see, 'door middel van geluid' can also mean 'by listening' (to/for a specific sound or tone).'
There's no word that means "help" in this sentence. "middel" means "means", as in "by means of".