Well, I'd say, if it doesn't happen very often, or you don't mind doing this for evry bubble, you can spell the sound that the animal makes in the bubble. For example, if you have a dog , make it do somehting like this:
bark bark bark bark bark
bark____HELLO!___bark
bark bark bark bark bark
But if you do it often, the best thing you can do is use a font that is remniscent of the animal (for example it being furry or dog/cat/etc. themed) or obscured as a way to make it hard to understand. Alternatively, you can watermark it with the sound the animal makes.