This is yet another case where WK tells me that I should know the reading because it uses on’yomi.
But…
… there are 2 ON readings for 楽 !? ラク is the one I’ve seen most often so far. However in this case, the correct pronunciation is ガク.
Is there a rule I am supposed to know about using one ON vs the other in this particular case that’s different from rendaku?
So this is why I keep getting it wrong. I keep typing そうらく and then WK says “This is the on reading, you should know this!” And I’m like, but that is the on reading that we learned…
But I just looked at all the vocab for 楽, and it looks like all the ones that have the がく reading have to do with music, e.g. 音楽 and 楽器. Idk if this is a rule for all of Japanese, but at least within the bounds of WK it can be applied.
As for the mnemonic being missing, がく is taught as an on’yomi reading for 楽 on level 8 when 音楽 is taught. So that’s why there’s no mnemonic on 奏楽.
That’s a good question about if there’s a rule for which on’yomi to use. I don’t think there is? But hotdogsuplex is onto something about がく being used in all music contexts. I’m gonna tuck that one away in the brain for later.
If there does end up being some kind of correlation, I think it would be worth mentioning, either in the 楽 kanji or 音楽 vocab. I know that would have saved me a lot of tears 
I’m not getting the logic. How does knowing that
音楽 uses ガク help in any way to pronounce 奏楽 when WK gives lots of other compound words where 楽 is pronounced differently?
Makes no sense to me.
Wanikani only tests that you know a reading, not every reading, so even aside from the disambiguation question, I think this isn’t a great assumption from Wanikani
I will pass this along to the content team. It does seem to be a valid correlation, at least on every music-related vocab on WK that uses 楽. If it helps, I am also full of tears.
No, I agree, it doesn’t help in differentiating. I just meant once an on’yomi reading is taught, then future vocab get the “you learned this, so you know how to read it” type prompts. That’s why that mnemonic is “missing,” so to speak.
For these specific 楽 words though, the music connection hotdogsuplex pointed out is a good one! I never noticed that! I passed that on to the content team to see if they can add that as a hint to differentiate these when to use which on’yomi.
Yeah you’re spot on. がく is pretty much always used for music vocab and らく is almost always related to something that has to do with comfort, ease, joy etc.