This group of verbs (ending in -ir) is quite small. However, there are different conjugations within this group, as you will see. Here again is true, what is true for many of the conjugations: the differences are in writing, not in pronunciation.
In this group we have two different stems, which always appears with perfectly regular verbs ending in -ir. The stem is, what is left, when the endings are taken away(-er, -ir, -oir, -re).
There are three groups of endings for verbs ending in -ir. The first has a irregular stem, the second group has a specialty: when the stem ends on an -t or an -d, it is omitted. The third group has endings like the verbs ending in -er.
Only the first group has a change in pronunciation.