Have you looked into and internal storm or laminated panel?

Maybe try to call Pella and see if they can remake the casement sash with laminated glass?

Pella said they do not make storm windows.
I was looking into getting a laminated glass panel but am just not sure if that would have the same effect as a soundproof window. Does anyone know?
I will try to contact Pella about their laminated windows - that may be a pricey option though, not sure. Does that involve just replacing the glass?

The laminated glass option from Pella would require a sash replacement as compared to a glass replacement.

Pella does not make a storm window but you can buy those from anyone.

The laminated glass upgrade in the existing windows would give you a noticeable performance upgrade but it would not be as large a jump or equivalent to a dedicated soundproof window or a laminated storm window combination.

It would, however, be the least visually obtrusive.