why wont mkv play on my phone

MKV is a container, not a codec — it's more like a box that can hold different kinds of video and audio streams inside it. Desktop media players (like VLC) typically bundle support for a huge range of codecs regardless of what container they're wrapped in, which is why MKV usually 'just works' there. Phones and mobile apps are much more restrictive: many rely on the device's built-in hardware decoder, which typically only supports H.264 or H.265 video with AAC audio — if the MKV file inside uses a different codec combination, playback fails even though the file itself isn't corrupted.

The symptoms vary depending on exactly what's mismatched: a completely unsupported video codec usually means the file won't open or play at all; an unsupported audio codec inside an otherwise-playable video can mean the video plays but with no sound; and some apps refuse to even attempt opening an .mkv file extension regardless of what's inside, rejecting it before ever checking the actual codecs.

Converting to MP4 with H.264 video and AAC audio — what this tool does — sidesteps all three failure modes at once, since that specific combination is close to universally supported by phone hardware decoders and mobile apps, rather than trying to diagnose which exact codec mismatch caused the original failure.

If you only need this to work on one specific device or app rather than everywhere, it's worth checking whether that app has an MKV-compatible alternative or a dedicated player before converting — but for general playback across whatever device you might use next, MP4 conversion is the safer, broader fix.