その話は前にも聞いたよ。is translated as "I heard that story before". I assume that the subject is implicit, その話 "that story" is the topic, 前 is "before" and 聞いた is the past of hear. What about にも? Why is 前 marked with these particles?
The も means "too; also; as well". 前にも means "(something happened) before, too (as well as now)". So you'd say 前にも when something that happened before is happening again now. Compare:
- その話は前にも聞いたよ。 I heard that before, too. / This is not the first time you've told me the story. / This is the second time you're telling me that.
- その話は前に聞いたよ。 I heard that before. / I know the story.
- その公園には前にも行ったことがあるよ。 I went to that park before, too. / This is the second time for me to go to that park.
- その公園には前に行ったことがあるよ。 I've been to that park.
に in 前に can serve as a time marker (as in
その時に) which would complete the "before" translation.
も and the ending よ give an additional sense of "already," perhaps multiple times before. The combined にも can express annoyance.
前にもあった：It has happened before.
前にも言ったけど: I've (already) said it before, but...