The mpv developers have deprecated some video and audio filters. SMPlayer 17.10 stops using those deprecated filters and is now using new filters.
Unfortunately older versions of mpv may not have those filters. This can cause playback without audio or video. A workaround is to turn off all video and audio filters. If you don’t have audio, turn off the audio equalizer in Preferences -> General -> Audio. Another workaround is to switch to mplayer by selecting mplayer as multimedia engine in Preferences -> General.
Of course the proper solution is to update mpv. Ubuntu users will find a recent version of mpv, which works perfectly fine with SMPlayer 17.10, in this PPA: