Believe it or not, this is something scholars actually discussed (ages ago), although the ruling is ijmaa (consensus) in our time.
Why would they bother? First, rasulullah said:
He who innovates something in this matter of ours [i.e. Islam] that is not of it will have it rejected [by Allah]. (Imam Nawawi's famous 40; Bukhari and Muslim)
Based on this hadith, which Imam Shafi'ee says is one third of faith--because it safeguards Islam from corruption by people--scholars understand two things:
- Any religious action (ibaadah) is haram until you bring strong proofs proving that Allah will accept it
- And its converse: any non-religious action (eating, sleeping, blinking, etc.) is permissible until proven guilty
They debated the microphone because it falls under the first category (religious actions), and as such, needs proof to support it.
Allah knows best about the actual reasons they concluded that it's permissible. It seems that since it doesn't really change the ibaadah, and you can probably make an analogy to the one who repeats the imam loudly so others can hear, it falls under the permissible rulings.
Since you mentioned "good bidah," that is a concept which only appears in ibaadah actions, not in natural actions. Some groups use it as a loophole to justify certain actions (such as mawlid), which are not part of Islam.