Dear toprk:
Thanks for the clarification and sorry for the misunderstanding,
I did some further test today, and here's the results.
Your topology:

- customer topology
- Picture 098.png (2.8 KiB) Viewed 9204 times
My test devices:1. Laptop(win7, with iTunes)
2. iPhone4 (iOS 6.1.2, with “Remote”, "Youtube" and some songs stored in iPhone)
3. AirPort Express (as the router and also connected with earphones as the output of airplay)
My topology:

- elsa topology
- Picture 099.png (4.02 KiB) Viewed 9204 times
Test case1:Use laptop to play a song that is stored in NAS, select the earphone as the output through airplay --> OK
Test case2:Use iphone to play a video on Youtube, select the earphone as the output through airplay --> OK, can hear the sound of the video
Test case3:Use iphone to play local songs with “Music” app, select the earphone as the output through airplay --> OK
Test case4:Use iphone to play a song that is stored in NAS with “Remote” app, select the earphone as the output through airplay -->
FAILED, the iPhone seems playing the song, but no sound from the earphone.
Test case5:Use iPhone to pair with the iTunes on my laptop, play the songs stored in the laptop --> OK
Test case6:Use iPhone to pair with the iTunes on my laptop, play the songs that is shared from NAS to the laptop --> OK
Due to the test results,
we also checked with our engineer, the pair between iPhone "remote" app and NAS only support HDMI and USB speakers now.
Please kindly pair to your Mac first then control Mac to play music from NAS to airplay speaker as the workaround for your application
(To play the songs from NAS to airplay speakers via "remote"),
and we'll try our best to implement directly airplay from iPhone to the speakers in the future.
Please feel free to let me know if there's anything we could help,
thanks!
Best Regards,
Elsa