Due to a fault with the sound for The Day of the Doctor, which only applies to a subset of systems listening on Stereo (more details by a Gallifrey Base user here), BBC Worldwide is replacing the second disc of the set.
This is also good news for fans who prefer to watch it in its original frame rate, 25fps, rather than watching the slightly slowed down 23.97fps version (personally, I can't tell the difference but hey!) since it will be provided in 1080i50.
Please be aware that this only applies to the 2D Collectors Edition Bluray disc, and not to the standalone 3D disc.
To get your disc replaced, pop the disc in an envelope, making sure to include your own address, and send it to this freepost address:
Freepost RTEK-EJGC-ZKGXThis freepost address only applies to UK and mainland customers.
BBC Worldwide Ltd
BBC DVD Support
33 Foley Street
The estimated time frame currently stands at around 1-2 weeks.