Asterisk 10 and IVR Announcement File Format

Using FreePBX 2.10 with Asterisk 10

I am using SLN16 format voice prompts successfully within my custom contexts (16bit 16Khz signed integer). However, when I I try to use one of these with the IVR Announcement I hear nothing. I’m lead to believe that the IVR announcement must be 8Khz 16bit, which is lower quality. ULaw encoding doesn’t work, either.

Do I understand this correctly? Why would Asterisk 10 support much higher quality recorded files, but not support them for the initial IVR Announcement message?