Possible Clues:

• Sound experiment with edible fungi builds muscle
• Reported attempt to sample mushrooms that are three-headed
• Muscle at the back of the upper arm
• Stumbles outside church - stretches out an arm
• That sounds like the mussel that's forearmed, perhaps
• Sound experiment with edible fungi produces muscle
• Dinosaur disregarding rodent and duck shows muscle
• Could make up the prices of mussels, by the sound of them, to have after tea
• Heard greengrocer's suggestion to build up muscle?
• Muscles shake ends of pectorals