polyvocally

The story

1. Proto-Indo-European*pleh₁-
2. Proto-Indo-European*polh₁ús
3. Ancient Greekπολῠ́ς (polŭ́s)
4. Englishpoly-
5. Proto-Indo-European*wekʷ-
6. Proto-Indo-European*wṓkʷs
7. Proto-Italic*wōks
8. Latinvōx
9. Proto-Indo-European*h₂el-
10. Proto-Indo-European*h₂élisder.?
11. Latinvōcālis
12. Middle Englishvocal
13. Englishvocal
14. Englishpolyvocal
15. English todaypolyvocally

From polyvocal + -ly.