I've no experience of communications in aviation but plenty in noisy environments. I dare say that some of the issues that result in someone 'declaring an emergency' might also result in additional noise. Noise cancelling technology can only do so much. The reduced bandwidth that you're left with will transmit some vowel and consonant sounds better than others.
The repeated vowel sounds in PAN, PAN and MAYDAY will have a clarity that I'm sure couldn't be achieved by 'I am declaring an emergency', even if the latter was uttered by Wallace Greenslade or an alumnus of Seagoon's school for announcers.
All I used to deal with was:
"Standby, LX cue 15"
"LX standing by"
"LX cue 15...go"
Zero peril, even if pyro was involved.
I'm not sure why you would choose to complicate what is, in essence, just a sound to gain attention for a very specific reason. If I've missed the point (other than contributing to thread drift), I do apologise.