Q:I have occasionally heard applause during Mass, for example after the homily or after a musical item, and have found it somehow strange and out of place. What should be my attitude to this?

Let me begin by saying that the Mass, or any other liturgical ceremony for that matter, is primarily directed to the worship of God. It is a time when we raise our hearts and minds to God in praise, thanksgiving and petition, and at the same time God showers His blessings and graces upon us.
The liturgy is thus a time for prayer and recollected silence and, yes, for singing out lustily in praise of God. But it is not a performance for the entertainment of those present. It is centred on God, not on the congregation. When, on the other hand, we attend a concert, a play, a sporting event or any other form of entertainment we naturally applaud from time to time, by way of showing our appreciation for the performance. This attitude and way of showing appreciation are not appropriate in the liturgy, which has an entirely different aim.
In his book The Spirit of the Liturgy, the then Cardinal Joseph Ratzinger summed up this criterion: “Whenever applause breaks out in the liturgy because of some human achievement, it is a sure sign that the essence of liturgy has totally disappeared and been replaced by a kind of religious entertainment” (p198).
While it is good that the congregation should be inspired by a homily, showing their appreciation habitually by applause has a number of dangers.
One is that the priest or deacon may unconsciously or consciously prepare the homily with a view to pleasing the people and eliciting applause, rather than seeking to build up the congregation in their knowledge and love of God. These two ends are very different and it would be sad if the homilist sought primarily recognition for himself.
Also, if the congregation have the custom of applauding from time to time, they can feel pressured to applaud in order not to offend the homilist, or they can judge the homily by whether others applaud or not and compare priests by the amount of applause they receive.
The homilist should seek to build up the people of God, and the congregation should listen to the homily with a view to taking away something helpful for their spiritual and human life.
For this reason the liturgical norms recommend a period of silence after the homily so that the people can reflect on what they have heard and assimilate it. Applause would disrupt this spirit of prayerful reflection
The same can be said about music. The purpose of music in the liturgy is to contribute to the beauty of the ceremony, and thus to enhance the glory given to God. When the music is particularly beautiful the people can naturally feel moved to applaud, as they would in a concert.
But then they should hold themselves back and remember that this is not a concert but an act of worship. They can thank God in prayer for music which truly lifts up their minds to Him and gives Him greater glory.
Naturally, there can be exceptions. In larger Masses celebrated by the Pope, people often show their love for the Holy Father by applauding when he comes in, and they sometimes interrupt the homily with enthusiastic applause. And in some cultures, applause is far more common in general than in the Western world, and so it may appear in the liturgy as well.
Moreover, there are some ceremonies in which applause is actually suggested by the liturgical norms. This happens, for example, in the rite of ordination of a priest or deacon, after the ordaining Bishop acknowledges that the person has been found worthy and is now chosen for ordination.
Spontaneous applause may also come and be appropriate when a newly ordained Bishop makes his way through the congregation blessing the people, or during the procession out of the church or Cathedral at the conclusion of an ordination ceremony. But in general, the liturgy should be quiet, prayerful and uplifting. Applause should not be a normal part of it.