(1) You should lower your hands at the time of dua and keep quiet.
ويأتى الماموم بقنوت الوتر لا الفجر بل يقف ساكتا على الاظهر مرسلا يديه .
(2) Do not offer witr salah behind those Imams who do salam at two rakah and offer one rakah separately. In such cases, perform with another jama’ah if you find some people and it is not improper, otherwise offer it alone.
(3) It is proved by hadith to do dua after fardh salah, and low voice is liked in dua. The holy Quran says:
ادْعُواْ رَبَّكُمْ تَضَرُّعاً وَخُفْيَةً [الأعراف : 55]
However, if namazis are not disturbed, there is no harm in doing dua loudly, but it is makrooh to make it a habit.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband