R1:ed750399a0ab

Make <audio /> work slightly better on devices

Summary: Ref T3887. `300px` is a little too wide on devices.

Test Plan: Viewed on a phone.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T3887

Differential Revision: https://secure.phabricator.com/D7157