Moving from Slackware 14 to 14.2 I forgot one or two details about how to get mplayer to display Japanese srt subtitles. At this time an essential bit is that mplayer needs to be explicitly pointed to a suitable font. I used the -fontconfig -font [fontname] solution, and put it in my .mplayer/config file.

Some options seem superfluous by now. I simply stopped experimenting when I got it to work.

subfont-encoding=unicode 
unicode=yes 
utf8=yes 
subcp=utf-8  
fontconfig=yes 
font='さざなみ明朝' 

Here is a copy of my mplayer config file