Update cheatsheets

mp4box

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# mp4box
+
+> MPEG-4 Systems Toolbox: muxe streams into MP4 container.
+> More information: <https://gpac.wp.imt.fr/mp4box>.
+
+- Display information about an existing MP4 file:
+
+`mp4box -info {{path/to/file}}`
+
+- Add an SRT subtitle file into an MP4 file:
+
+`mp4box -add {{input_subs.srt}}:lang=eng -add {{input.mp4}} {{output.mp4}}`
+
+- Combine audio from one file and video from another:
+
+`mp4box -add {{input1.mp4}}#audio -add {{input2.mp4}}#video {{output.mp4}}`