Yes, they exist. There are five (5) B350 motherboards, that I know of, which has an Optical (S/PDIF Out) slot at the rear panel:
These are:
Asus ROG Strix B350F Gaming (Uses FX S1220A Audio Codec)
Gigabyte AB350 Gaming 3 (Uses Realtek ALC1220 Audio Codec)
MSI B350 Gaming Pro Carbon (Uses Realtek ALC1220 Audio Codec)
MSI B350M Mortar (Uses Realtek ALC892 Audio Codec)
MSI B350M Mortar Arctic (Uses Realtek ALC892 Audio Codec)
While several other B350 motherboards (about 9 or 10 models) don't have the
actual rear panel slot, but has a
2-Pin or 3-Pin S/PDIF Out header on-board (where you can purchase a separate expansion slot with an Optical (S/PDIF Out) slot, like this:
These are the following:
Asus Prime B350-Plus,
Asus Prime B350M-A,
Asus Prime B350M-E,
Asus Prime B350M-K,
Gigabyte AB350 Gaming,
Gigabyte AB350M HD3,
Gigabyte AB350M D3H,
Gigabyte AB350M Gaming 3, and
Gigabyte AB350M DS2.
Among all the motherboards mentioned, I'd suggest either the
Asus ROG Strix B350F Gaming or the
MSI B350 Gaming Pro Carbon. Both these motherboards have the highest number of power phases (Asus having 8; MSI having 10) for better OC voltage regulation for your Ryzen 5 1600.
For the RAM, the 3200MHz pairs well with Ryzen CPUs, if your budget allows it. A 16GB (2 x 8GB) kit is best to take advantage of dual-channel memory speeds. Though AMD Ryzen, being new, is a bit picky on RAM compatibilities/timings, the Corsair Vengeance LPX DDR4-3200MHz should work with the above boards.
MSI QVL:
https://www.msi.com/Motherboard/support/B350-GAMING-PRO-CARBON.html#support-mem-2
Asus QVL: http://dlcdnet.asus.com/pub/ASUS/mb/SocketAM4/STRIX_B350-F_GAMING/ROG_STRIX_B350-F_GAMING_SummitCPUDDR4_DRAM_QVL20170520.pdf?_ga=2.160307261.1678855619.1498979407-1428616915.1495995095