The most simple way I can think of would be to de-mux the audio (snag the audio seperate off the video). edit the audio in an audio editing program, then re-mux it. While audio editing replace an existing audio or mix in the desired audio, leaving the length of the audio and video the same.
Using Direct stream re-muxing the video would go untouched, as the audio (which also could be direct streamed) would be added back to the set.
Another method would be to "append" create a video using (or converted to) the exact same bitrate, the exact same codec and the exact same everything. If you managed to get all the the technical details of the AV exact an append could be done (again) without touching the original (direct stream). Most programs would not respond to this added chunk being audio only, that is why you create a video, even if the picture is blank.
The above things can be done with Ffmpeg and virtualdub, the ease of accomplishing the task would be highly dependant on the original codecs used and complexity of the original AV stream. Example: VBR audio streams are not the same bitrate, giving you only error on append. Multitrack and 6.1 audio and all that fun stuff can make editing the audio much more than a valentine gimick.
(That means if you have not already started , you wont have it done for this years valentines day:-)
Many of the programs and methods that would make this process more simple, would re-code the video, By re-coding the video your in control of all the Technical details of the video, and appends will go as planned, muxing and mixing in new items go as planned because everything is re-coded the same. Re-coding high bitrate videos, using proper codecs and settings would not degrade the video/audio as much as it does when they are already chrunched severly for webplay or webtransport.
Another method to keep from destroying the original quality of the video, would be to compress the final output way less, that way additional losses and artifacts are not added (as much) to the video because your not attemptting to squeese it back in the same can it came out of. Just check the ability of the playback device to handle the datarate , and it would still be fine.
A playlist and a skip button would not be the same at all. But if you personally made a short video (instead of simple audio) tested a slight of hand skipping before presenting,, and delievered it well, it might be appretiated the same, and be a lot less work/fails.