totally agree, its hard to find a balance between this points because maybe one has made his music and sounds during the jam and put a lot of efforts on it but he made a normal game, on the other hand, one put his efforts on the game design and used premade music and sfx and made a strong game.
the second person is probably the one that have a higher chance to win, since its a "game" jam (btw there is OST jams which is about making the best music).
but in my opinion both of them did efforts even the one who used premade assets, because that one has put a lot of efforts on the game design, while the other one has put a lot of efforts on composing the music/sfx, so in both cases both of them put a lot of efforts.
but when it comes to rating/judging the audio between them, even if its hard to judge which one, in my opinion the one that should win in that category is the one that have his music fit the game style, but its still hard to pick which one of them >.<