A few years ago, the original version of Musicpleer was closed - because it was sued for copyright infringement by the authorities. Since then, a lot has happened in the music market. New pages were created and also Musicpleer clones. They all offer the same thing. A free MP3 search engine which displays title, image preview and video length. Then you can choose a video and download it for free on your smartphone or computer. The services are all almost identical fast and it usually costs nothing. The services such as Musicpleer is financed with advertising which can be seen on these websites (PopUps, banners, etc.)
On most of these platforms you can not only download YouTube videos but also from platforms like Soundcloud, Deezer, etc. The most popular is of course the YouTube download. The quality of the MP3 or MP4 files depends strongly on the source file. If a video has already been loaded on YT in poor quality - even the best converter cant improve the audio quality.