Urad dal used whole in thoran/rasam - how to correctly prepare?
I believe raw urad dal is meant to be split urad dal as mentioned by @Athanasius .
Any dal you use make sure it is split and not whole otherwise i will not temper properly.
Whole dal becomes too hard or burnt in oil.
For me the dal becomes crunchy and it is supposed to be a bit hard(not teeth breaking hard but like bits of corn seeds in pop corn).
It would never become completely soft as you are tempering it. Rather than soaking in water i would suggest roasting them in oven or low gas heat before you use it to temper if your dal stays to hard for you.
Like Daliya (Roasted chana dal) that we use in Idli chatney, gives nice crunch to your chutney. I hope this helps.