[ubuntu_studio] Low sound output on Macs

I have installed Ubuntu 16.04 on a 2010 MacPro (Intel) and a mid-2011 iMac. In both instances, the output volume is low. I have everything turned up to 100%, and I can hear sound, but it is quiet at best. I searched the threads for a similar issue, but could not find any. How do I fix this?

Thanks