I'm not entirely sure, but I remember vaguely that 5.1 was only available when shooting AVCHD. With XAVC-S 4K, I guess there is no 5.1 option, only stereo.
EDIT: I found the following in the manual.
Movie recording format:
XAVC S (XAVC S format) Video: MPEG-4 AVC/H.264 Audio: MPEG-4 Linear PCM 2ch (48 kHz/16 bit)
AVCHD (AVCHD format Ver.2.0 compatible) Video: MPEG-4 AVC/H.264 Audio: Dolby Digital 2ch/5.1ch Dolby Digital 5.1 Creator*1
MP4 Video: MPEG-4 AVC/H.264 Audio: MPEG-4 AAC-LC 2ch *1 Manufactured under license from Dolby Laboratories.
