@Sara - Looking on another ifixit procedure, I found that the speaker wires are taped to the battery, so I suspect that the speaker was not properly plugged in after reassemble, or that something happened to the wires during battery removal. Hopefully the wires weren’t cut or stretched.
Here is what the XPS 13 teardown procedure had to say:
“….After that, it still isn't a perfectly clean getaway—the speaker wires are strategically taped to the battery in a half dozen spots, which we'll have to liberate to free the battery. (Admittedly, it could have been a lot worse.)
These same wires are trapped under the battery screws—so speaker replacement means battery removal. It looks like the XPS has some layering issues.”
ref: Dell XPS 13 Teardown
I hope this helps.