I think reading out loud helps because if you're just saying it in your mind you're not use to saying and pronouncing the words. You probably think you are, but you really aren't. I have personal experience of this because I use to learn Portuguese very early in the morning and would just say the words in my head so I didn't wake anybody up. Now, I learn later in the day and feel more confident with speaking words. I noticed that I really struggled to pronounce the words when I first started speaking them though. Even though I thought I had mastered them