After using them for a while, I would say the strops only get good when they are not slippery at all.
Let them get dry, crusty and black. Use them far, far longer than you think you should, and then they are starting to get good in my experience.
I know it sounds counterintuitive, but give it a try.
As for sound – I would not go by it at all – I have no idea what they should “sound like”.
