Verbs of perception (such as “hear”, “see”, “feel”, and others) can be used both with the -ing form (e.g. “hear doing”) and with the infinitive without “to” (e.g. “hear do”). The meaning is different in each case, however.
The “hear do” form describes a completed one-time action, whereas the “hear doing” form implies a continuous or repetitive action. For example:
If you heard the book “hitting the floor”, then the book either hit the floor several times, or it was a longer continuous process making a noise. If you heard the book “hit the floor”, it was a one-time short thud. Another example: Imagine you saw a group of children who were jumping repeatedly into a swimming pool. Then you would likely say:
There’s nothing wrong with using “saw the children jump”, but that would imply that the children did it only once (or that you saw them do it only once).
A repetitive action can often be thought of as a longer one-time action, which gives us a certain freedom of expression. For example, both
I heard her playing the piano.
make sense and can be used completely interchangeably.