I can sing their songs without even looking at the lyrics.
I know all their songs by heart.
この場合、次のような言い方ができますよ。
ーI can sing their songs without even looking at the lyrics.
「彼らの歌は歌詞を見なくても歌える」
ーI know all their songs by heart.
「彼らの歌は全部、空で覚えている」
例:
I really love this band, so I can sing their songs without even looking at the lyrics.
「このバンドが大好きなので、彼らの歌は歌詞を見なくても歌える」
ご参考まで!