I do both, depending on where I am, or how I'm feeling. Each has worked very well for me. If my thoughts are too loud then I'll force myself to say it out loud so that I can penetrate the noise of my mind! 

Also, sometimes I speak LOUDLY and AFFIRMATIVELY like a command. The question you're asking, to me, is basically, "Is it effective to speak to my subconscious mind out loud or quietly?" 

I know both work because of the following: 

I was having trouble oversleeping EVERY SINGLE DAY. In the past I had experiences where I'd be able to tell my Subconscious mind to wake me up at a certain time, and she would do so.

But at this particular time in my life I would tell my subconscious, "wake me up at 8:00AM", in my mind. Without speaking. It just wasn't working as it had in the past. I was doing this all without an alarm clock set.

I finally gave up on wanting my subconscious mind to wake me up. Basically if you don't know, "Hey Siri" on the IPhone, is a feature that allows you to literally say, "Hey Siri", and then the iphone wakes up from hibernation to the sound of those words and allows you to speak to her, and she will do what you ask. You can program certain tasks such as, "Set a reminder,..." or, "Wake me up at...", or ,"Send a message to mom"... etc.

So I was in the completely different room than where my phone was charging. The phone was charging in the kitchen and I was laying down in my bedroom. I yelled, "Hey Siri, WAKE ME UP AT 8:00AM!"

I guess doing that in a loud, authoritative manor impressed upon my subconscious mind that I wanted to wake up at 8:00AM. My subconscious mind ended up Waking me up at 7:59, right before my alarm started to sound, as if just to show me that she was still willing to do what I asked, so long as I did it in the correct manor.