Mplayer version in SMPlayer 21.1; files playing too fast

[brittney24]

Using SMP smplayer 21.1, Using MPlayer SVN r37540, in Mint 18.1 64bit. How does the SVN number relate to mplayer release versions - or is there a direct correlation?
When I check for mplayer version in terminal, it shows MPlayer 1.2.1 (Debian), built with gcc-5.4.0 (C) 2000-2016 MPlayer Team.. I guess from 2016? Fairly old?

I believe the latest mplayer release is 1.4.
But that may not be the version included in the SMP package, rather one included w/ the Mint distro.

Other issue: I've read very old posts - here & other - about mplayer playing some mp4 vids a bit too fast. I'm not sure they apply now.
A common suggested fix was enter the fps value in mplayer config. But this issue is rare for me, so I don't want to enter fps=29.nnn in mplayer options that affects all files, many over 30 fps.

Any more recent ideas on the cause here?

I've seen several mp4 files that didn't show under View > File Properties 1 or several specs, for time, fps, bitrate, etc. Some of those may just have corruption.
But I've also seen some mp4 files missing some of the same types of data (under View > File Properties) that played audio & video just fine.

Also, the "Audio/Video Auto synchronization" nor entering an A/V Sync Correction factor doesn't seem to do much on files out of sync more than just 1 or 2 sec. Pos. & Neg. values I entered may have changed the audio / video speeds, but never got close to being synched.
It may be bcs of a partially corrupted file. Is there a tutorial on using the 2 sync methods, on files not missing lots of meta data?
Thanks.