Ogu,
1. Speaker sound. Iinstalled stock ROM a few weeks ago, and frankly speaking I think stock sound is even lower. I use CM11 with Williams Kernel which makes a possibility to increase sound volume using configuration files. Now the sound is enaugh for me.
2. Files. You need to have rooted device and a recovery (simple use CWM) installed. So, that what you need if you dont have a recovery is download zip file, take boot.img from it and flash it using flashtool. Then you restart your phone and call CWM pushing vol up number times. You have to install whole zip in CWM recovery, what takes just a 1-2 minutes. Then you can install PA-GAPPS, this package contains google apps from nexus devices. It is not neccessary, as you can install everything you need from a google playstore
3. There is CMdelta which is a differential installer, but I don't use it. Instead of it you can use OTA installer (similar to stock). But, please remember, you dont need to install every nightly.
4. It depends on nightly version. The overal performance is worse after some of them. Sometimes batery drain quickly. For example 18082014 nightly make batery loos power quickly, but next nightlies solved it. No I can use my device all day without charging, what is offcourse worse result than the stock. I use my phone very intensively, today I took it from a charger at 7am, now is 4pm and I still have over 60% left (my phone has used almost 40% during this time).
And, I don't play any games, so I cannot confirm a game performance.
5. My cpu is ovbercocked to 1832MHz an the highest temperature I had was about 37 Celcius. Normaly the phone is cold. Otherwise, you can downclock for having better batery life or lower temperature. Batery, as I said, after 9 hours I still have over 60% to use.
