For Miresua, I pick a mixture of the letters from the Basque and Finnish words. But how should these letters be pronounced? Good question. As to be expected, Basque and Finnish pronounce things differently.
After some consideration, I've decided to add a new letter to the Miresua conlang. This will be an exception to my alphabetic mixture word construction.
I'm adding the letter ü -- that is u with an umlaut or two dots over it. It’ll be pronounced like u in the French word tu or the German letter ü. This is a reasonable addition. Miresua already uses two letters with umlauts -- ä and ö -- courtesy of its Finnish derivation. The ü sound is already present in Finnish; it's the letter y in Finnish. Estonian, which is a language closely related to Finnish, uses ü, instead of y, for this vowel. There is also a Basque dialect that makes use of an occasional ü.
Tentatively, my new rule is this: if the Finnish source word contains two "y"s, they can be replaced by a ü. I'll be modifying several existing Miresua words to take advantage of this change.