RAW - no, and definitely not at +16

First, quibbling about the attack bonus if Sound Strike were a natural attack. The full wording of the natural attacks section of the Polymorph subschool rules states (emphasis mine):

In addition to these benefits, you gain any of the natural attacks of the base creature, including proficiency on those attacks. These attacks are based on your base attack bonus, modified by your Strength or Dexterity as appropriate, and use your Strength modifier for determining damage bonuses.

For a ranged attack, the attack bonus is BAB + DEX modifier of the subject of the spell (which is Personal). The Muse gets +16 because she has BAB of +8 and a DEX of 27 (giving a modifier of +8). A character who has cast Fey Form does not benefit from those modifiers, they have to use their own.

However, the more important question is whether this is a natural attack or not. The Muse appears in Bestiary 5. "Natural Attacks" have a lengthy entry in the Appendix 3: Glossary on p295 of the same book - the entire section is too long to reproduce here, but some relevant excerpts are:

Most creatures possess one or more natural attacks (attacks made without a weapon). These attacks fall into one of two categories: primary and secondary attacks. Primary attacks are made using the creature's full base attack bonus and add the creature's full Strength bonus on damage rolls. Secondary attacks are made using the creature's base attack bonus -5 and add only 1/2 the creature's Strength bonus on damage rolls.

…

The Damage Type column refers to the sort of damage that the natural attack typically deals: bludgeoning (B), piercing (P), or slashing (S)…

Some fey, humanoids, monstrous humanoids and outsiders don't have natural attacks. These creatures can make unarmed strikes, but treat them as weapons for the purpose of determining attack bonuses, and they must use the two-weapon fighting rules when making attacks with both hands...

The takeaways from this are:

• natural attacks are using strength to deal bludgeoning, piercing or slashing damage.
• some fey do not have natural attacks, but can attack unarmed (as per the melee attack listed for the Muse). Note that the mention of fey specifically is in Bestiary 5 (the book the Muse is from) but is not in the linked PSFRD entry.

Based on this, the sonic strike is not a natural attack.

Some of the comments and answers are noting that sonic strike is not listed as a special attack or special ability, hence it must be a natural attack. While I understand that arguing about writer intent turns into an opinion-based exercise in futility, it needs to be pointed out that in this instance we already know that the entry for sonic strike is incomplete. There is no range listed! RAW, sonic strike is a direct fire attack with infinite range that can be used at line of sight with no range penalties! (The same issue exists with the Flytrap Leshy on p156 of the same book, its acidic spittle can go on ad infinitum too - pretty good for a CR4 monster. Maybe that's why it doesn't seem to have made its way into the PSFRD.) For comparison, the Piercing Hymn of the Choral Angel also does 4d6 sonic damage but is a defined power with a range of 90'.

TL;DR - sonic strike is not a natural attack and the GM needs to work out its range and any other details in their campaign.