Hi leoric,
On my bedroom computer I'm not using a receiver/preamp, so I need to decode in software and send audio out via my analogs. For me the following works:
Recent stable versions of AC3File and ffdshow installed.
I'm not sure if this changes anything, but I happen to be using the directshow audio in JRiver (due to some other software).
Now in MC (I'm using 15), Options > File Types:
AC3 Audio (ac3), DTS Audio (dts)
Playback Method: JRiver audio engine (using DirectShow filter)
Source Filter: AC3File
Other Filters: ffdshow Audio Decoder; JRiver Bitrate Monitor filter
In the ffdshow audio properties/configuration Codecs, I happen to be using "liba52" for AC3 and "libdts" for DTS. I'm not saying these decoders are the ones to use (in ffdshow), but they seem to be working fine for me.
If you are using S/PDIF and sending the ac3/dts audio out to your receiver for decoding, then make sure that in ffdshow's "Output" you have AC3 and DTS checked in the "Pass-through (S/PDIF,HDMI) section. Since the decoding is external, after setting the passthrough double check your AC3 and DTS decoding settings in the codec section above since now you want the audio sent out without any decoding on the computer.
I realize your setup and configuration is different and that you are using WASAPI but thought maybe seeing above may help in some small way. Good luck!