Best practices for audio and animated gifs?

I got some feedback on a new WBT the other day and I'm not sure how to handle it:

On most slides, there isn't any audio or narration. There are a few slides with animated videos, and these videos have music playing in the background. But otherwise, no audio.

However, I do have some animated gifs/small videos on some other slides just as visuals to illustrate text (e.g. a small ringing telephone with a block of text next to it), and the feedback I got was that some users were confused by these slides and turned their audio all the way up to try and hear what was happening -- and then, hearing nothing, forgot to turn their volume back down (and got blasted the next time they got to a slide with an actual video on it).

 So my question is: Are there any best practices for marking on a slide that there isn't any audio? I'll probably just change the animated gifs/small videos to plain jpegs, but I thought I'd see if there's an option I'm not thinking of.