YOU SAID:

I've heard some of their songs before

INTO JAPANESE

彼らの歌をいくつか聞いたことがあります

BACK INTO ENGLISH

I've heard some of their songs

INTO JAPANESE

彼らの歌のいくつかを聞いたことがあります

BACK INTO ENGLISH

Have heard some of their songs

INTO JAPANESE

彼らの歌のいくつかを聞いたことがあります

BACK INTO ENGLISH

Have heard some of their songs