Thanks, JohnK. I think I solved the mystery...
A lot of the reviews of my player (DVP-NS315) say it can play DVD-A, but I now think that is a mistake. The manual clearly says "does NOT play DVD-Audio".
Oh well...
The Joni Mitchell disk is DVD-A, not SACD, as it turns out... but it has THREE modes recorded on it -- DVD-A, Dolby Surround (2.1) and Dolby Digital 5.1. When I look at the receiver status while playing it is definitely Dolby Digital 5.1.
M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8
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