Northing have been done for that directly in Openshot.
Perhaps in MLT but I got some doubts. Which version are you using on the both ? It can be explain this issue.
Other track for that, I know that Alsa had the emulation for oss, and it can explain too this support. BTW, I was thinking that the oss project was dead. Happy to see that he exists always and go onward.
Thanks for the feedback.