A lot of qualifiers need to be roped around this question to offer an answer. First of all we have to ask when and where do you have in mind? A sermon from a parish priest is much more likely in the later Middle Ages (say 13th century on) than earlier, and much more likely in a town parish than a rural one. I’ll focus on the 13th century forward since it’s the period I know best. Second, I’m not sure what you mean by a “king’s parish.” Though there was a lot of give and take during the Middle Ages over who controlled a parish, the crown or the church, by the thirteenth century kings generally would have little interest in the daily spiritual goings-on—whether folks were hearing sermons—in the average parish. Parishes were of interest to them only insofar as they were an economic entity that generated revenue. But that’s not really the thrust of your question so I’ll leave the details undeveloped here. Also relevant here: there were thousands of parishes in medieval Europe (over 3,000 in England alone) and sources to specifically answer your question are pretty thin, so by and large we can speak only of things parish priests should preach rather than what they actually did preach or even whether they preached at all. Actually, historians still disagree about how frequently sermons got preached on average. More recent studies suggest that sermons were a lot more frequent in even rural parishes than we had assumed even just 30 years ago.

So, all that said, what could parishioners expect to hear in a sermon? Admonitions and religious instruction are a convenient way to divide the subject matter. Admonitions encompass the “legalistic” obligations that episcopal statutes (those issued by a bishop whose diocese contained dozens of parishes) and provincial statutes, issued by an archbishop who presided over a number of dioceses, ordered every parish priest to announce to his congregation at regular intervals. These admonitions were things like instructing parents to be sure to have their children baptized and the ritual words to use if there was an emergency and a priest wasn’t available. They were admonished to have their children confirmed when a bishop passed near their parish. They were reminded that they were obliged to receive the sacrament of penance and take holy communion at least once a year in the Easter season. They were reminded that they had to pay their tithes (a 10% religious tax imposed on all that they produced). They were cautioned not to associate with excommunicates or harbor robbers and thieves. They were cautioned to respect church property, especially by not profaning the churchyard where parishioners were buried by holding dances, or markets, or games, etc. there. Mothers were cautioned not to “overlay” their newborns by accidentally rolling over on them and smothering them in the communal bed. Sometimes parishioners had long lists of crimes that warranted excommunication read out to them. Banns of marriage were supposed to be announced three Sundays in a row so that anyone who knew of an impediment to the marriage—for example, that the bride and groom were too closely related—would have time to report this.

Religious instruction was the second element of the parish sermon. Practically, godparents were instructed to make sure their godchildren learned the Creed, the Lord’s Prayer, the Hail Mary, and how to make the sign of the cross. But moral instruction, how to live a Christian life, was supposed to be the backbone of the sermon. Parishioners were instructed about the Ten Commandments, the 12 Articles of Faith contained in the Creed (i.e., the meaning of the Trinity, the Virgin Birth of Jesus, his Passion, his Resurrection, etc.), the seven sacraments, Seven Works of Mercy. They might hear stories about the saints associated with their parish and about the most popularly regarded saints (e.g., the Virgin Mary, St. Nicholas, Mary Magdalene, early Christian martyrs). They might hear stories of miracles that illustrated the doctrine of the Real Presence (i.e., the actual Body and Blood of Jesus) in the consecrated Host and wine. These might involve people seeing a little child in the Host, or a Host that dripped blood or tasted of flesh in the mouth (ick!).

The best sermons would be illustrated by “exempla” (“exemplum” singular), which were exactly what they sound like: catchy little stories that were examples depicting some moral lesson (follow the link for examples of exempla!). If parishioners were really lucky a Dominican or Franciscan friar might pass through the parish. These friars were trained as preachers—ordinary parish priests were not—and were known to give the best sermons. Friars like Bernardino of Siena (+1444) or Vincent Ferrer (+1419) gave sermons that could last 3 or 4 hours and attracted crowds of thousands in the towns where they stopped. But even parish priests not trained as preachers had access, if they were so conscientiously inclined, to bound collections of sermons meant to cover the feasts (Advent, Christmas, Lent, Easter, etc.) and saints feast days scattered liberally across the year. There were dozens of these, often in the vernacular as well as Latin, available by the mid 1200s on.

Sources: all of this fits under the category of “stuff I know” (imagine how fun I must be at parties!) but you can find a classic discussion of sermons in the intro to Richard and Mary Rouse’s Preachers, Florilegia and Sermons: Studies on the Manipulus florum of Thomas of Ireland. More recently, try Siegfried Wenzel’s The Art of Preaching: Five Medieval Texts and Translations (he’s the dean of medieval sermon studies) or Katherine L. Jansen’s The Making of the Magdalen: Preaching and Popular Devotion in the Later Middle Ages