I know this doesn't answer your question, but have a look at GStreamer pipelines. They take the basic idea of shell pipelines, but add typing, format negotiation, M:N connections, etc all while giving you optional control over low-level details (which demuxer?), but also high-level abstractions (just show me a window with a preview). Once prototyped on the CLI (gst-launch-1.0[1]), they're also very easy to start using within an application through the framework (gst_parse_launch[2]), where you can e.g. iteratively add things like a volume slider. You can also access most of FFmpeg's codecs through libav.

[1]: https://gstreamer.freedesktop.org/documentation/tutorials/ba...

[2]: https://gstreamer.freedesktop.org/documentation/gstreamer/gs...