In recent years, I’ve been happy to hear more Spanish in vocal recitals and concerts along with the usual Italian, French, German, and Latin (the latter especially in the choral world). It makes sense: for a small investment of time and effort, you can reap significant rewards in terms of repertoire choices and potential audience growth. For this guide, I’m going to assume you know the basics and instead focus on what kind of Spanish I recommend for my own music.
My mature works in Spanish only involve texts by Americans—using the word broadly to include North, Central, and South America—and I endorse a kind of generic Latin American pronunciation. The most important feature in this regard is the seseo, where z and soft c are pronounced [s], in the manner of most Latin American speakers, not [θ] as is more typical in Spain. For example, corazón should be sung [koɾason], not [koɾaθon].
The pronunciation of ll varies widely across the Spanish-speaking world, but I support the practice of yeísmo for singing: that is, pronouncing both ll and y as [j].
I’m a little conflicted about b and v. Technically, they are to be pronounced exactly the same: [b] after a pause or nasal consonant, and in all other positions [β] (a sort of cross between [b] and [v] where the the lips don’t touch each other or the teeth). But I do sometimes hear [v] among native Spanish speakers in the US, which I don’t think is wrong even if it is a result of English influence. Languages have always influenced each other; English, among others, would not exist without that process. And for the purpose of singing, I think [v] can provide a nice emphasis without being plosive; some singers have used [v] on volar (“to fly”) in my song La nostalgia and I thought it was very effective. But I respect the choice to use [b] there too if you prefer to be more traditional.