Have you checked your Windows sound mixer settings to ensure everything is working at 16 bit 48 kHz sampling rate? You check it by right clicking on the speaker icon in the System Tray, choosing Playback devices, then selecting two items: one will be the Speakers themselves so double click that item on the list, then the Advanced tab, make sure it's set for 16 bit 48 kHz, then close that dialogue box and double click the virtual audio cable and verify it's set the same. DSD+ won't decode most anything at all with any success unless that aspect is taken care of up front, so to speak, as that is the "working" mode for it to read/decode the signal it's being fed. This might be a slightly different procedure with Windows 10, I can't say for sure, I don't run that monstrosity and never will so... somewhere in the Windows 10 audio mixer you should
be able to configure the audio chain to be 16 bit 48 kHz as required.
It can be rather complex for someone that's completely new to using this type of monitoring setup even in spite of SDR# being fairly easy to use (can be downloaded, extracted, updated with the install-rtlsdr.bat file to get the DLL files installed, then ran and tuned to a frequency in under a minute basically) and DSD+ isn't all that difficult to use if you simply extract the distribution archive and then run it with dsdplus.exe (which lets it decode whatever comes its way).
Adding a virtual audio cable into the mix complicates things depending on which one you use as there are two popular ones: the Virtual Audio Cable commercial product (works well enough but is way overcomplicated for most people and in that group I would include anyone that just wants to decode some digital format with DSD+) and then VB-Audio Cable (which technically is donation-ware meaning you can use it freely but the author does ask for a donation of your choosing if you find it useful and believe me it's incredibly useful). VB-Audio Cable takes a few seconds to set up and literally once it's installed you honestly don't have to do anything else to make use of it - the Virtual Audio Cable commercial product (again these are two different applications/drivers from two different authors but many people get confused because they're both "virtual audio cable" software) is vastly more complex and can even create up to 256 of them if you really ever had such a need to do that sort of thing.
So, the basic signal path goes like this as long as the Windows sound mixer is set correctly for 16 bit 48 kHz (listed as "DVD Quality") if you're using SDR# + a virtual audio cable + DSD+:
- SDR# tunes the frequency where the digital format is being broadcast, so you'll need to ensure two things with that application: 1) the audio output is selected as the virtual cable input and 2) you need to ensure the Filter or any audio filtering of any kind is not enabled in SDR#: there's a checkbox in Audio side panel that says "Filter Audio" and that must be unchecked/deselected to pass the proper baseband audio to DSD+ over the virtual audio cable. Any kind of audio filtering going on will wreck any potential decode and you'll never get it working. You should also set the SDR# volume at about 40-50%, it can be adjusted later on when you learn how to get the levels working properly as some transmissions simply don't meet the necessary decoding thresholds but 40-50% should be workable for most systems in use.
- DSD+ then reads the baseband audio passed over the virtual audio cable and will decode it either in a narrow mode meaning you specify precisely what you wanted decoded (like only decoding P25 traffic or only DMR) or the default which is that it'll attempt to listen for any of the supported digital formats and decode them automagically as they hit that virtual audio cable.
Because I've done this setup a few hundred times over the past 3-4 years I can cold-clean setup SDR# (with the Zadig drivers as well), VB-Audio Cable, and DSD+ and get a working decode of most any digital format that DSD+ supports in like 2 minutes tops on most any Windows machine out there and I have done that for several people in my area that needed some face-to-face help. Now considering you're just getting into this it's understandable to reach some sticking points along the way but the best advice I can offer is don't give up because when you do get it all working and hear that decode for the first time you'll more than likely have the same reaction most people do:
"I'll be damned, it works...
Check everything as just described especially the Windows sound mixer settings and then verify you're piping the audio out to DSD+ from SDR# and that it's unfiltered as it needs to be.