Consistent Audio Levels Across Tracks (image included)

I am editing many audio recordings to help people heal. Many, like the image below, are rather quiet on average and they sound fine when you turn up your volume all the way. How do I wisely turn up the volume and have all the various audio files generally be around the same average volume?

I know I could manually increase volume for the entire track using the heads-up display, or drag up the yellow level line. But is there a more professional way to do this and keep the resulting audio levels consistent across tracks?

Thank you!