It is not currently possible to mute the sound in all of the Osmo apps, but it's something we're looking to change in the future. Some Osmo apps do have the option to mute sounds, voices, and music, so we recommend checking in the Settings menu for each game to verify.
If you want to have additional sound settings added to a specific Osmo game, please fill out our feedback form, and we will be happy to take your feedback into consideration for future versions of our games.
For reference, you can also use the volume buttons on your device to turn the sound down in any app.