Yes, Gionee A1 Plus is a true 4G VoLTE mobile phone and you can use VoLTE without any problem.
Gionee A1 Plus has dual sim card slot of different size. You can use one micro sized sim card and another nano size sim card in this phone.
Yes, Gionee A1 Plus does have a hybrid sim slot.
Yes, Gionee A1 Plus has stereo speaker. One speaker is located on the bottom of the phone and ear-piece also works as speaker and thus you will get excellent stereo sound output.