This is an o!f issue because borders are applied into the individual
sprites of the container via masking, rather than being isolated to the
container itself. In this case, it'll be applied to the "flash" sprite,
which is using additive blending, causing further issues.
Also refactors the whole method to generally clean things up. One more
important fix is setting the frequency on the channel before starting
playback, so avoid the frequency potentially being adjusted after the
playback is already started.