Sometimes it is nice to just grab ingredients from the pantry / freezer and throw things into a pot for a quick "homemade" soup. Typically quick soups like this utilize a mixed bag of frozen vegetables. Unfortunately, I can often taste that the vegetables were frozen. Is there a way to mask this frozen flavor?
What you are describing is often worst when the vegetables are thrown together without care as to what vegetables will do well stewed for a while, and which vegetables only need to be heated through and will suffer if they are cooked longer.
A great example of that is in the case of typical "frozen mixed vegetables". Carrots are never nicely tender in mixtures like that. Lima beans are downright chalky and peas are total mush. If you want to use frozen vegetables, avoid mixtures.
Pick the vegetables that you actually enjoy, and add them to your soup at a time appropriately distant from the time that you actually want to serve the soup. So bring lima beans to a hard simmer before you add corn, bring that to a hard simmer before you add frozen potatoes, add frozen carrots, bring that to a hard simmer before you finally add peas just before you serve - your soup will be better than emptying a bag of mixed vegetables into broth.
I realise this is old but I'm surprised no one has said this yet.
Soaking/thawing the vegetables in a salted water solution for 2 hours will remove the freezer taste and ensure the vegetables properly marry with the soup/broth/sauce.
Separating mixed veg as previously stated can help because of different cooking times but the veg is already parboiled to different consistencies to equalise the cooking time of the mix. A premium mix might be better at equalising the cooking time but it is brand specific.
If your frozen veggies taste like they were frozen then that's the issue to solve. Decent frozen vegetables which have been stored properly should not have any off flavors at all. If, on the other hand there are odors in the freezer, or it's not storing things cold enough, or you haven't properly re-sealed the vegetables after you took some out then that could lead to things tasting a bit off. Also, they could just be old, nothing lasts forever.
So, make sure you've got the right storage temp, and maybe clean the freezer out thoroughly. Stick an open box of baking soda in to control odors, seal your vegetables properly, and get rid of them if they are more than 6 months old.
If you still want to use them you'll have to cover it up with strong flavors like a nice hearty stock, garlic, chili, etc.
I cannot imagine what the specific taste is that indicates vegetables were frozen, assuming you are using a product with reasonable quality.
You may optionally choose to saute the vegetables before adding them to the soup to get some additional flavor development (this will reduce the total time they need to cook in the soup).
Also, make sure you are seasoning your overall soup sufficiently, as frozen vegetables, much like fresh ones, have no salt.