Oh you poor thing to have several episodes a night!?
I experienced this once whilst about 23 wks pg with my son. I'd woke from a nap and my eyes were open but I couldn't move or speak, and had the same ringing noise as well. I remember trying to scream for help but nothing happened, it was the scariest thing i've ever experienced.
Have you ever mentioned it to GP? Maybe try a natural sleeping remedy so you are nice and relaxed before bed, and a hot bath maybe?

Have just had a quick google and this site has some good tips: http://www.spis.org.uk/

They say:

• Keep to a regular schedule. Go to bed and get up at regular times. Eat your meals at regular times.
• Get some exercise, although not too close to bedtime.
• Avoid sleep deprivation - make sure you get enough sleep.
• Find ways to reduce stress in your life.
• Try to avoid sleeping on your back.
• Know that you are not alone - there are many other sufferers alongside you.
• Finally, relax! You are not dying, you are not going insane. Sleep Paralysis is not known to be harmful.

Lisa x