Supercarvitation is fast (800km/h), but not faster than the speed of sound.
Sounds in sea water (1500 m/s) travels much faster (x5) than in air (331.3 m/s)
So, Passive sonar should still be usefull, the main problem however, is that you won't be able to outrun the torpedo again
Though increasing the speed of a torpedo does theoretically confer a significant tactical advantage to the combatant employing them, supercavitation has its drawbacks as well. In short, naval tactics have evolved to the point that the use of supercavitation is not of undisputed benefit, and the navies that employ it tend to be those that have not perfected sophisticated guidance systems for their torpedoes.
Naval combat frequently occurs over significant distances; the Mark 48 torpedo, the staple torpedo of the US navy, has a range of five miles with unconfirmed reports of ranges in excess of twenty miles. The maximum speed of at least 32 miles per hour, (unconfirmed reports indicate speeds upwards of 60 mph) meaning it can reach its maximum range in a matter of minutes (ten to twenty, depending on the figures used). A supercavitating torpedo traveling at 230mph would still take over a minute to reach a destination five miles away, and around six minutes to reach a target twenty miles away. This is more than ample time for a target to dodge. But whereas conventional torpedoes are capable of homing in on a target using either wires connected to the launching ship or active sonar, the nature of supercavitation precludes either method of guidance. The supercavitating engine would sever any wires attached to the torpedo, and the bubble of vapor surrounding the torpedo both enables it to travel at very high speeds and prevents the use of sonar. Supercavitation also produces an incredible amount of noise, which alerts the target to both the torpedo and the location of the launching sub.
A common submarine tactic is to quietly launch a torpedo, but not to activate it until the firing sub has moved a few hundred yards away, then activating it and guiding it to the target. Supercavitating torpedoes are only capable of being "dumb-fired" directly from the launching vessel, which would immediately reveal its position, a lethal mistake in modern naval warfare. As the torpedoes produced by the American navy - and sold through arms contracts to allied navies - have incredible guidance systems, interest in supercavitation has been rather muted in the Western naval community.