I've been trying what you said, and using max x256 TC. But there are still problems, and I'm wondering if there are even bugs in the game.
...so there I was, skimming along the surface at x256, when boom-ping-whizz-pop - "we're under attack, sir! We're taking damage, sir!"
FFS! Almost all sections of the hull were damaged.
Luckily, I'd taken a game save earlier in that sector, so I was able to recreate the scenario easily to analyse what had happened. There was a Flower Corvette waiting for me, with its searchlights on! I replayed from the save file up to the point just before I was in range of that FC. I checked the hydrophone - NOTHING! No sound at all - not even the background sound of the deep. I asked the sonar man for the nearest warship contact. "No sound contact" was his reply. And yet, 2 mins later in real time, I was under attack.
Why does the hydrophone background sound disappear? Is this a bug? I notice that other sounds also disappear - the periscope up/down sound goes, and the electric engine sounds in some cases.
Can you explain how I could be 2 mins away from an attack, but for nothing to be heard on hydrophone/sonar?
Just to show I wasn't making it up...