A video I am creating won't have voice-over but only captions. Does anyone have any ideas on a start-up jingle so viewers know that their audio is working and it's intentional that there's no voice-over?
Depending on how retro you want to get, you could do something like the old Outer Limits tv show used (cf. https://www.youtube.com/watch?v=FCcdr4O-3gE). If your segment is short enough, a simple voiceover saying there will be no more voiceovers would not only be sufficient, but satisfyingly ironic. Or, if you don't want to use any voiceover, you could use a fanfare to draw attention then explain in captions that only captions will be used.