Can I turn this around and ask a simple question here:
Why does it matter what anyone here thinks about this?
We don't know you and your family....
The reason I ask this is because I'm wondering if you are wanting validation of your opinion or that your relationship with your family is just peachy, and I kind of feel like you should already know that within yourself.
"Yelling" is sort of arbitrary-- because as you, yourself have stated-- your family doesn't feel it is a negative. Your mom has given you confirmation that things are a-okay and your husband sounds like he agrees.
That said, here's what I'd do-- go forward doing what works for you, DO NOT factor MIL's opinion into things and DO NOT compare her relationship with her family to the one you have with yours. She has a different perception than you do. Perceptions are merely that, ways of seeing things. The funny thing is that you BOTH feel you have good relationships with your family, in the loud or quieter styles you have them in....
Can you let it go?
As for if I overheard someone yelling at someone else....I would try not to make any assumptions about their relationship at all. I can't see both parties, can't see the expression on the second party's face, don't know what they are yelling on and on about-- frankly, I would probably be walking away or trying to focus on something else. I don't find the experience of being near anyone yelling down the phone at anyone else pleasant. Can't say I would jump to the conclusion that the relationship is fine and dandy-- but really, who would?
Know your own truth and don't give this too much thought. Frankly, don't even give MY answer too much thought. If something is sticking in your craw, it means that your perception might benefit from some reexamination (including your opinion/feelings about your MIL). Reexamining things is always good, but to ask a bunch of strangers on a forum~ be careful what you ask as you may not like the responses.... (and remember, we are just people, no one knows you... SO KNOW your own truth!)