A major 2nd below C is a Bb, which is a whole step. Maybe just try singing down do-ti-te and that would be the right note.

@Brendan K: That's how I approach small descending intervals, I sing down by half steps. Then I double-check by singing back up diatonically.

When it says, major 9 down instead of major 2 it can be, that you're singing (and hearing) it an octave off or the algorithm is detecting it wrong .. but I encountered this mostly the other way round .. so, e.g. 2nd upwards got interpreted as a 9 upwards, or major 2nd downwards as minor 7 upwards .. especially, if it's close to the lower end of my vocal range .. then I try to focus on singing with as low overtones as possible, e. g. mmmh with closed mouth 😅
btw. to get the major 2nd downwards, I mostly go down the perfect 4 to SO and then semitone upwards to the LE and on to TE .. that's the way, I got secure .. now, I'm trying to get the interval directly, the way it's asked, without any detours via solfeggio-steps ..

PS: and of course, major 2nd below C is Bb

@Thomas Kumar your theory that I was hearing it in the wrong octave was totally spot on! Kicking myself for not thinking of that.

Also I'm sorry everyone about the D vs Bb mix up 😅 my brain went the wrong direction when giving an example.

A M2 below C is Bb. Consider the Major 2nd as a WHOLE STEP (or Tone) - above or below. So, if the Interval Barks game is asking for descending M2, just go a whole step below the Root note, in this case Bb.