Hanafi
The sources give no view for it here.
كون المؤذن حرًّا
هل يُستحب أن يكون المؤذن حرًّا؟
Each school’s answer
Hanafi
Not stated in the sources
Maliki
Not stated in the sources
Shafi'i
Not stated in the sources
Hanbali
Not stated in the sources
The texts
ب - الأَْذَانُ، وَالإِْقَامَةُ، وَالإِْمَامَةُ: ٥٦ - يَجُوزُ أَنْ يَكُونَ الْمُؤَذِّنُ وَالْمُقِيمُ عَبْدًا عِنْدَ الشَّافِعِيَّةِ وَالْحَنَابِلَةِ. ثُمَّ قَال الْحَنَابِلَةُ: إِنَّ عَلَيْهِ أَنْ يَسْتَأْذِنَ سَيِّدَهُ. وَذَكَرَ ابْنُ هُبَيْرَةَ أَنَّهُ يُسْتَحَبُّ أَنْ يَكُونَ الْمُؤَذِّنُ حُرًّا (٢) .
«الموسوعة الفقهية الكويتية» (Kuwait) entry «رق» vol. 23 p. 41
From the schools’ own books
Maliki
The sources give no view for it here.
Shafi'i
The sources give no view for it here.
Hanbali
The sources give no view for it here.
What bears this mark is in our words, our standard wording; the books’ texts stay as they are.