Exegesis:

pantes hoi akousantes ‘all who heard,’ without specific object; this is either the shepherds, cf. “them” (Phillips, La Sainte Bible: Nouvelle version Segond révisée) or what the shepherds told, cf. “it” (Revised Standard Version, An American Translation, Zürcher Bibel), preferably the latter.

peri tōn lalēthentōn hupo tōn poimenōn pros autous lit. ‘about the things that had been told by the shepherds to them.’

Translation:

Some possibly necessary changes of the sentence structure are, ‘the things the shepherds had told them astonished all people who had heard them’ (Sranan Tongo), ‘all who heard what the shepherds told wondered (at it).’

Wondered at, see on 1.21.

What (or, the things/matter) the shepherds told them, or ‘the words of the shepherds (to them),’ ‘the shepherds’ story’ (Javanese).

Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.