the annoyingly undetailed Segmentation fault error message
If this is happening something is probably broken, and if you have really replaced the executable repeatedly, it is likely something it links to -- a library, but obviously not too critical a one or else you would be having this problem with other things as well (or, it could be the broken bit is in a common library, but it is an obscure bit).
By "broken" I mean the file has become corrupted. The only other possibility, since you are using stretch ("but the default repo is jessie") is it is some subtle mismatch in pieces that has gone unnoticed (which might happen particularly if it is a relatively unique mismatch of jessie and stretch pieces).
If you want to try tracking the problem down (and I cannot promise it will work),
apt install gdb, then run
gdb mplayer, it will take 10 seconds or so to get to a prompt, then enter
run ... where
... is the rest of the commandline you would usually use with
When the segfault happens, the prompt will come back. Type
backtrace. The first time you do this, you may get a lot of
in ???, but you may also get a message to install some debugging packages (I'm not positive the Debian/Raspbian version does this; if not things just got too complicated). Unfortunately those can sometimes be quite large. You can then try the backtrace again and get more details.
If you edit that backtrace into your question I can hazard a guess at what is broken.
However, if this is happening more widely you might as well re-install; random corruption of system libraries is not something that can easily be fixed.
Another possibility, since
mpv has the same problem, is to look at the output of
ldd $(which mplayer) and
ldd $(which mpv) and see what the most obscure things they have in common are (not
libdl, etc. -- again if you post this in the question I could hazard a guess). You should then try replacing the packages that provide those libraries. If you are not sure use the Debian package contents search.