True audio is AMD's implementation of tensilica audio DSP's, they licenced it from Tensilica. It's something you need hardware for that AMD added in. PS4 added it in, XO didn't(though they seem to have added their own version). It's incredibly likely that Nvidia can add their own implementation of the DSP's into their own chips.
It is not software that is locked away detecting a Nvidia card and refuses to work. It's something that needs specific hardware that AMD chose to add into their cores. I'm not 100% sure Nvidia can licence it, but afaik they can implement whatever audio dsp's they like and I can't see game dev's going out of their way to make better audio that only works on AMD cards(and consoles).
http://ip.cadence.com/ipportfolio/tensilica-ip/audio
the software, say astoundsound or whatever, and the tools made to help sound engineers create the sounds for in game aren't directly tied to AMD. There is little reason Nvidia can't implement their own version after licensing the IP from tensilica and have access to the same audio pathways from a game engine in the future. It will be called something different to trueaudio, but that is just what AMD call their implementation. It's not locked out from Nvidia, they just don't have hardware physically capable of actually doing it right now.
That is something that is a vastly vastly different situation to taking software that can easily run on the other companies hardware and doing everything possible to lock it out. Hell Nvidia will lock out Nvidia features from Nvidia cards if an AMD card is present.
There is also no arguing that TressFX is hardware agnostic, it is, it doesn't magically run much slower on Nvidia cards, it runs effectively the same speed on both hardware(there will always be differences but there isn't anything significant at all). Game dev's are both free to use it, free to change it, free to show their code to Nvidia, Nvidia is free to see the original TressFX code. It is completely hardware agnostic and AMD has made absolutely no attempt to block it out at all and it could easily have done so. They actively chose to allow it to run on any card, Nvidia almost always chooses the other option when given the choice and have gone out of their way on multiple occasions to make something run worse on AMD hardware and gone out of their way on multiple occasions to cause a game to run slower on ALL hardware just in the hope it hurts AMD performance more(like tessellating the living crap out of completely flat or completely hidden textures).
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