What is the best answer to early mutas in a TvZ game?

Since you mention the 7 minute mark, even without scouting, you should be able to find a way to put down an engineering bay about 5 minutes in and that will give you plenty of time to get up a few turrets up around the 7 minute mark.

Turrets are very good versus a low number of mutas (which they will have around the 7 minute mark, 5-7 max).

With the early engineering bay you can easily drop 100 minerals and gas for a quick +1 attack upgrade which will help versus zerg.

Of course you should always scout and not spend needless minerals on the turrets if they are not going mutas. The engineering bay is still helpful for the upgrades.

As a zerg player i can say that a small groups of marines and a few well placed turrets will stop my mutas.

Mutas are expensive and zerg players usually bank some resources before. This can mean that when the mutalisk attack come, early on, zerg are light on other units. Meaning if you scout for it, prepare your defense at the base and position yourself for a counter attack you can damage/break him.

The zerg player can prepare for this with static defense (Spine crawlers) and good positioning of structures. This holds of your attack so he can return the mutas and hatch new units before you do enough damage.

So scout and be prepared to not commit to the attack if he is prepared.