Summary:

An Avel of a parent during the year of Aveilus may not participate in a Seudas Bris Mila that is taking place outside of his home. If the Avel is having a Seudas Bris Mila take place in his home, then he may participate in the meal. In all cases, the Avel may participate in the actual ceremony of the Bris even outside of his home, including the light refreshments given afterwards, so long as he does not remain for the meal.

Baal Bris-Bris of child within Shloshim-Mohel and Sandek: If an Avel [father or mother] is having a Bris for his son take place during Shloshim, he may wear Shabbos clothing until after the Mila, and may participate in the meal even during Shloshim, if it is taking place inside his home. The same applies if the Avel is a Mohel or Sandek.

Q&A

See Halacha B for various Q&A relevant to this topic, recorded regarding Shloshim, and the same law applies within the year of Aveilus. The Q&A include the following topics

1. Participating in a Vacht Nacht
2. Participating in a Shalom Zachar
3. Participating on Shabbos
4. Grandparent participating
5. Brother or sister participating
6. Yeshiva student participating in Yeshiva
7. Guest participating in house of host
8. Participating for purposes of Parnasa
9. Waiter participating
10. Avel being a Kvater and participating
11. Rabbi participating

May those people who are permitted to participate in the meal do so even if there is music playing?[1]

This matter is disputed in Poskim.[2] Practically, they are not to participate in the meal while the music is playing.

May an Avel enter the Seuda to simply wish Mazal Tov to the family?[3]

It is permitted to do so long as there is no music playing at the time he enters, and he does not participate in the meal.