Question: What are the things that invalidate salat as they are considered outside intervention?
ANSWERSome of them are as follows:
1. Saying “Amin” to the du’a of someone other than the imam you are following invalidates salat. If a person performing salat next to you recites Surat al-Fatiha aloud and if you say “Amin” at the end of it, your salat will be invalid. Similarly, if a group of men starts to perform salat in jama’at when you are performing salat individually, your saying “Amin” when the imam of that jama’at recited Surat al-Fatiha will invalidate your salat because he is not your imam. If a person comes and says “May Allah forgive you your sins” when you are performing salat and if you say “Amin,” your salat will be invalid.
The sound produced by a loudspeaker is not the imam’s voice from a scientific perspective as well, so it invalidates salat to say “Amin” to the sound produced by the loudspeaker.
http://www.myreligionislam.com/detail.asp?Aid=6236
