First, if your azaleas are in pure lava and have not had a problem previously, it’s a miracle. Lava is pH neutral, and you need an acidic environment for the azalea’s roots to take up iron and manganese. This is why we use kanuma, and dress the soil with yamagoke moss.
Second, if you wait until they are dry, you’re killing them. Azaleas are very sensitive to root temperature and moisture, and they should never be allowed to dry out. Moist is the key condition to shoot for.
If it were me, I would get some kanuma. Then I would remove the tree from the pot and dunk it in the peroxide solution recommended by Osoyoung [who offers his usual good advice]. I would cut away any roots that appear to be rotted, but otherwise leave the root mass unmolested.
I would then gently re-pot, on top of a layer of large-particle kanuma at least a finger-width deep, and surround the root mass with a similar width around the edges using a smaller particle size. I would pot it so that the soil came up to the base of the trunk, leaving none of the roots exposed to air. I would then put a bamboo skewer into the soil, and check it daily to see if I needed to water [thus taking the guesswork out of the process].
I would also prune back any branches that appear to be desiccated or are brittle, until I found green wood, and cover the cuts with the orange cut paste for azaleas. I would then place the pot in the shade, and not fertilize until the tree was healthy.
Then I would light a novena candle, pray for the best, and start shopping for another tree, since even with all of the above, the odds are probably 4:5 against.
That’s just me. Your mileage will almost certainly vary.
Good luck.