Warbucks is currently missing his "taking damage" instrumental voice line and doesn't make any noise when suffering damage.
Warbucks instrumental voice line's (except his Battlecry voiceline) also sounds abnormally quiet compared to the rest of the cast. This might not be a bug but rather a conscious design decision of his character. Either way I still believe it may be a bug and worth evaluating.
Play as Warbucks and take damage to find he makes no instrumental voice lines on taking damage. To notice his quiet voice examine any object and compare it to other characters to find he sounds far quieter than the rest of the cast.