If with "GSX for P3D", you mean the 64 bit version, they use exactly the same identical code for audio, that is the OpenAL 64 runtime, which is one and shared between MSFS and P3D 64 bit.
Nothing in our audio code does anything differently depending on the simulator used, the only difference is that 64 bit and 32 versions use a different OpenAL runtime, but in that case, they both shouldn't work, assuming your audio drivers had a problem with 64 bit OpenAL.