My understanding of the term bidah is things which are inventend in the religion. So how would you classify loud speakers which are used for calling adhan.
To find out more about the meaning of bid’ah and guidelines concerning it, please see questions no. 7277 and 10843.
With regard to giving the adhaan via loudspeakers, there is nothing wrong with that because it is a means of making the adhaan reach the listeners, and the means come under the same rulings as the ends. It is required for the muezzin to raise his voice so that the people can hear him, so whatever is a means to this end is also required.
“Using modern weapons of force and taking care of them is included in the aayah,
‘And make ready against them all you can of power…’
[al-Anfaal 8:60 – interpretation of the meaning]
and using means of defence against deadly weapons is included in the aayah,
“Take your precautions”[al-Nisa’ 4:71 – interpretation of the meaning],
and the ability to travel by sea and by air is included in the aayah,
“And Hajj (pilgrimage to Makkah) to the House (Ka‘bah) is a duty that mankind owes to Allaah, those who can afford the expenses (for one’s conveyance, provision and residence)”
[Aal ‘Imraan 3:97 – interpretation of the meaning].
All of that and other things are included in the commands to use all means of strength and jihad.
Similarly, making voices and useful opinions reach distant places by means of telegraphs, telephones, etc. is included in the command of Allaah and His Messenger to convey the truth to mankind. The ability to convey the truth and beneficial words through a variety of means is a blessing from Allaah, and developing and inventing means that serve both religious and worldly interests is a kind of jihad for the sake of Allaah.”
(From a khutbah by Shaykh Ibn Sa’di when a loudspeaker was installed in the mosque and some people denounced that. Majmoo’ah Mu’allafaat Ibn Sa’di, vol. 6, pl. 51).
Similarly, using the Internet to convey beneficial knowledge and call people to Islam is one of the most useful means of achieving the great aims of sharee’ah.
We ask Allaah to help us to obey Him. May Allaah bless our Prophet Muhammad.