Is it permissible to signal with the hand or raise the voice to alert someone during prayer?

There is no objection to a worshipper gesturing to alert someone else to something, and the prayer is not invalidated by it. The Prophet, peace and blessings be upon him, gestured to those who were praying behind him while standing to sit down. There is also no harm in a worshipper raising his voice with some recitation with the intention of making others understand, such as raising his voice with the verse "Enter it in peace, secure" with the intention of granting permission to enter.