What can I substitute?

Cream of chicken, cream of celery, cream of broccoli, or just fresh cream. You can add a beef bouillon cube or beef stock base if you want.

My mom is full blood Swede, well technically Swede-Finn. No cream of any sort in how it's made. I firmly believe her version is traditional.

Then cook something else. If you still insist on cooking this, make a Béchamel sauce and use that. If you don't know how to make a Béchamel or white sauce, learn.

Make your own white sauce and use that in place of cream of anything soup.

I would saute some onions along with the butter before the flour is added for the sauce and make more of a cream of onion for your sauce.

But if you must use a creamy condensed soup I would go with cream of celery

I'd go with cream of celery.

I think it is in the nature of a cream sauce, ie, a milky or creamy liquid base plus seasoning and thickener. It is just a basic white sauce.How you make it attractive enough without the mushrooms is your part. If dealing with canned soups any cream soup would be okay, so long as it agreed with your conception of Swedish food.

There are different cream soups... such as cream of broccoli, cream of celery, cream of chicken, etc.

anything else but mushrooms

Cream of Onion Soup, Cream of bacon Soup ,

Just look at a different recipe. I've never seen a a recipe for Swedish meatballs that called for cream of crap (sorry, that's what we jokingly call it in my house even though I actually love cream of mushroom). In fact, I don't think I've ever seen a Swedish meatball recipe that calls for mushrooms period.

A sauce/gravy made from scratch will be made with a roux (butter/flour) and then have some combination of cream/milk plus stock/broth and seasonings.