For me the portal in the Nether spawned above a lava lake and I spawned inside the blocks I was supposed to be standing on, thus took suffocation damage and fell into the lava lake.
I got a similar problem to this in 1.6.2 and the problem was that the crative audio configs was set to 7.1 instead of headphones or 2.1. Changed that and everything is fine
For me the portal in the Nether spawned above a lava lake and I spawned inside the blocks I was supposed to be standing on, thus took suffocation damage and fell into the lava lake.