Here’s a video from Scott Devine, of Scott’s Bass Lessons, that has an ear-training exercise for bassists – although it can probably be used with most instruments. It basically calls for the person practicing ear-training to sing solfege or just sing something in the same pitch as the interval that’s being played. The video focuses on notes of the major scale, but it can, obviously, be altered to any scale, and any pitch or tone.
I’m glad that Scott’s about as good a singer as I probably am, which means I’ll have to practice this in my closet at night through a handkerchief, and its fun to hear that his wife actually IS a singer. That Coursera course that I’m about halfway through now has included ear training for 2 intervals each week for the past 3 weeks now. I haven’t gotten to watch the Lesson 4 videos yet, so I don’t know if it will continue. We covered Major 2nds & 3rds in Week 1, Perfect 4ths & 5ths in Week 2 and then Major 6ths and 7ths in Week 3. I don’t see any intervals in the Lesson 4 titles.