There are some points here:
1. Let us assume that the horn/relay is faulty, the Maruti service will blame me since the car is still under warranty and I have installed the audio system myself.
2. Will that noise I get in the speakers affect my audio system in any way? I presume it will not.
3. Non audio question: For argument's sake, if I remove the audio system and take the car to the service station, there will be no noise naturally when the speakers/HU/amp are removed. How will I prove that the horn/relay is faulty?