I don't know much about the FPC internals, but usually in compiled languages case statements are better for the optimizer because:
1. case statements ignore the order, if statements don't. This means that the compiler can reorder the statements for optimization
2. case statements are simple ordinal matching statements, meaning they can not contain complex logic
3. case statements basically form a jumping table, on consecutive values of a small range, this can therefore be optimized as such
But after all, as you are playing sound files, efficency is none of your concern (the loading and decoding of the file from disk is so much slower than any of your code logic, meaning you probably should go for the solution that gives you the clearest, easiest to read and maintainable code. Usually, if applicable I think these are case statements.