This is all normal.
You cannot use other mod's resources in your mod by reference.
E.g. for sounds, you can use numbers from 0 to 54 (vanilla)... which map to the same indices in all mods. You can also override them using these indices... and this will override them globally. If multiple mods override them, the last one loaded will win.
If you use custom sounds (55+) they will map into a different index for EACH mod.
For example 1055 for piratez and 2055 for your mod and maybe 8055 for some other mod... based on the mod's size and order of loading.
In other words, index 55 is only virtual and always maps to something else.
That's why when you wanted to use piratez's sound 55 (which is actually 1055), you have actually used sound 8055... which didn't exist, because you didn't define it in your mod (as 55).
Little confusing, I know.
PS: same applies to extraSprites