。
She sat on a bench, humming an old tune in a low voice.
Literal
She [topic-は] old tune [object-を] low voice [in-で] hum [while-ながら] bench [on-に] was-sitting.
ハミングする is one of the many English loanwords adopted as a noun and turned into a verb with する — the same pattern that gives 勉強する, ジョギングする, ノックする. ~ながら hooks two simultaneous actions onto the same subject: humming AND sitting at the same time. The で in 低い声で is the means/manner で ('with a low voice'), while the に in ベンチに座っていた marks where she came to be seated — に rather than で because 座る focuses on the resulting position, not the activity of sitting. The whole sentence has a quiet, reflective rhythm, like the opening of a short story.