Question:
If I'm praying alone, can I say Shmoneh Esrei out loud if no one is there to hear? It brings about more kavanah for me to pray out loud.
Answer:
Hello,
The halacha is that if by not saying the words out loud you will not have adequate kavana, you may say the words out loud. However, if you can have adequate kavana without saying the words out loud, and saying the words out loud merely helps you have "more" kavana, you should not say the words out loud.
Practically speaking, nowadays people find it very difficult to have kavana. Therefore, if you feel that you can have kavana without saying the words out loud, you should not say the words out loud. However, if you feel that without saying the words out loud than you will not have kavana, i.e. you will not concentrate on the meaning of the words, than you should say the words out loud, provided that as you are at home and will not disturb anyone else's davening.
All the best
Sources:
Shulchan Aruch O:CH 101:2, Mishna Berura 101:8 and Biur Halacha "Ve'im".