Add licence file.

src/sound/LICENCE.md

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+# Sound effect licences
+
+The following sound effects have been licensed from Pixabay, under https://pixabay.com/service/license-summary/.
+
+- `raise_hand`
+- `reactions/cat`
+- `reactions/clap`
+- `reactions/crickets`
+- `reactions/dog`
+- `reactions/generic`
+- `reactions/lightbulb`
+- `reactions/party`
+
+### Other
+
+The following sound effects have been originally created by Element.
+
+- `blocked`
+- `end_talk`
+- `start_talk_local`
+- `start_talk_remote`
+- `reactions/rock`