Is the prayer of one who completes his prayer behind a congregation that began immediately after the congregational prayer ended in the mosque acceptable, while another congregation formed with a louder imam's voice, and is it correct to leave the first congregation and join the second?

What you did by establishing a second congregation in a mosque where a first congregation had already prayed is a matter of dispute. If the mosque does not have a resident imam or is a mosque frequented by travelers, then establishing a second congregation is not disliked. However, if it is not frequented by travelers and has a resident imam, the majority of scholars consider it disliked, while others hold that it is not disliked. As for initiating a second congregation when the first congregation is already present, it is disliked, and you are not permitted to interrupt your prayer to join it. Your prayer is valid, and acceptance is with Allah.