The i5 is a fully customized Android Digital Audio Player. We have clarified repeatedly that a lot of Android service that is not related digital audio playback have been sealed/removed. For this reason, while we support installation of third party applications, but we cannot guarantee all applications will be fully functioned in i5, especially non-audio related features.
Most android mobile phone can playback Youtube with perfect video/audio synchronization. That's because the Audio and Video decoding (and amplification) are done in an integrated chipset, This is not the case of i5. The video signal of i5 is decoded by the MCU but the audio signal will output from the MCU to a comprehensive DAC and analogue circuit, and I believe this is the main selling point of i5.
Since the audio and video signal are decoded under a very different signal path, so there is good chance that the result will not be synchronized correctly. To achieve synchronization under this signal processing condition, we need to develop a DSP function called Lip Sync. This is not tremendously complicated, but does required to downmix all audio signal to a specific bitrate (which will defeat the purpose of Hi-Res audio support completely) before the DSP platform can compute the delay required to correctly synchronize the audio and video signal.
In a nutshell, i5 does not have this DSP capability, and Cayin has no plan to implement such a DSP function in order to support youtube playback.