Original 'Sound of Music' Actress -- Live Special Was 'Painful to Watch'

Gretl Von Trapp from the original "The Sound of Music" movie was left "mystified and disappointed" after last night's live remake on NBC ... and says Carrie Underwood TOTALLY BOMBED!!

Gretl a.k.a. Kym Karath met up with former co-star Charmian Carr (Liesl Von Trapp) to watch the special last night ... but their excitement deflated once "Sound" started.

During the special Kym tweeted, "Mystified & disappointed so far by SOM special. So far only happy with Stephen Moyer" ... then followed up by saying, "Must admit some scenes are actually painful to watch."

That's when Karath kicked things up a notch by roasting Underwood's performance adding, "Love Carrie Underwood but this role is just not right for her. She is lovely her voice is beautiful but acting is wrong."

For what it's worth ... the special brought in HUGE ratings for NBC with 18.5 million viewers.