This link:
http://www.copyright.gov/fls/fl102.html says "The distinction
between 'fair use' and infringement may be unclear and not easily defined".
Applying the four factors to the video for Aunt Martha doesn't really help
clear things up any.
1. Clearly a use that is not "of a commerial nature". You might even be
able to stretch and say that it is for "educational purposes" (learning how
to use a computer/WMM2)
2. I'm not sure exactly what "nature" means here. Would there be a
difference between a currently popular song and something obscure?
3. One song off a CD, so maybe 10%. Could be less if you buy just one song
from iTunes. But the site says "There is no specific number of words,
lines, or notes that may safely be taken without permission"
4. Using a song in a video for Grandma Jones probably isn't going to make
the song any less valuable. You might try arguing the likelyhood of Grandma
Jones buying/not-buying the song based on it being included in your video.
But this is all moot: even if a laywer (or some court ruling) said that
using a song in a personal video is "fair use" and not "infringement", you
STILL can't legally unlock the DRM. I wonder if there is a specific ruling
on point (fair use, not DRM), as you say nobody is "...likely [to be] sued
for making a [personal] video...", until a Judge makes a ruling this is all
mostly interpretation and opinion (and I am not a lawyer).
Dan