Which QuickTime Codecs Do I need for MKV videos?

Do you have https://www.perian.org/ installed? VLC doesn't require it as far as I know. I have NO experience with El Cap, much less QTP7 with it.

I am trying to watch an MKV video on QuickTime Player 7. The video works fine, but I get no audio and a pop-up that says I need "additional software" to run the video properly... though it doesn't bother to tell me what I need (as an added note, this movie plays just fine on VLC Player; I want to use QuickTime player so I can step through it frame-by-frame if I need to).

QT 7 does not natively support AC3 audio. VLC and MPlayerX are usually the preferred Mac players here for MKV files with 5.1 AC-3 and/or 5.1 DTS audio. Both support "passthrough" of multichannel bitstreams for external decoding and internal stereo down mixed audio output. However, while VLC does not support the QT player arrow convention for frame stepping of content, the MPlayerX app does.

If all else fails, use Movavi Video Converter 4 to convert the file to .mov format, to then view in Quicktime.