Always use 'said' as the verb to describe the act of someone speaking.
Neutrality - 'said' is a neutral word, it disappears to the reader so doesn't clutter the page unlike, say, expostulated.
Redundancy - "Why do you like cats?" she asked. There's a question mark, so it's obvious he's asking a question.
Physical impossibility - "I love cats; they're so cute," he giggled. Giggling makes a noise, but not one that forms itself into intelligible words.
When to break the Rule
Volume - you may want to indicate if someone is shouting 'he yelled' or speaking quietly 'she whispered'.
Tone - muttering is different in tone and attitude from whispering, or speaking quietly for that matter.
Information - 'I'll have another drink,' he slurred shows us his state of inebriation without spelling it out as in 'I'll have another drink,' he said drunkenly.