Came across this old thread and maybe I can add a bit to the good information it already contains.
Sonobuoys
The signal level was a factor but so was the character. The main sound heard was usually described as a "swishing" from the propellers. A "beats per minute" calculation could be done to give an indication of the U-boat speed and changes in speed. The sound was created by the cavitation (air bubbles forming and collapsing around the blades) but this faded with the increase in pressure at greater depths making it hard to track a deep U-boat.
In trials in benign conditions there are reports of tannoy announcements being picked up. Various mechanical noises of differing authenticity were reported in numerous accounts but analysis of after-action reports frequently concluded it was operator imagination.
Check this site out if you want to hear real and training sonobuoy recordings. (subsequently seen someone else already posted the sound files)
Historic Naval Sound and Video
Ultimately the non-directional sonobuoys were not great successes in WW2 despite 150,000+ being made. They were more of a prototype and results were weak. Post-war developments turned it into a key tool in ASW.
Aircraft types using sonobuoys
From US and RN carriers the Avenger was by far the major user. Possibly the only one in the Atlantic for the USN?
Both the USN and Coastal Command used sonobuoys on B-24s so the issue of flying a large multi-engine patrol plane was not just a post-war challenge. There are reports of B-24s laying a pattern beneath cloud cover at 500 feet. Initial instructions to the crew basically placed the bulk of the decision-making on the first pilot. He was clearly overburdened and revised doctrine passed more of the sonobuoy aspects to the operator leaving the pilot free to concentrate of flying, a busy enough task at low level and in poor visibility. Sonobuoys were fitted in land-based aircraft such as Lancasters and Halifaxes but the B-24's dominated. Catalinas and Cansos also featured.
Incredibly the old sturdy Swordfish biplane carried sonobuoys along with radar, rockets and/or depth charges operating from RN CVEs. With the open cockpit, even with good headphones, it must have been very difficult. Just to add to the fun the operator's seat faced forwards but the receiver was placed behind so he had to twist round to alter the tuning dial. All in bracing Atlantic weather.
Weapon use
All forces used sonobuoy information as adequate tracking information to justify launching a Mark-24 (often in pairs) close to a buoy returning a good sound. With its homing ability there was thought to be sufficient chance of a hit. The USN did use sonobuoy information to drop depth charges but the RN always felt the accuracy was far too weak to make that effective. All aircraft would look to track a U-boat and home in close by surface escorts to commence an ASDIC search and, if necessary, continue a hunt to exhaustion.