Convert audio and video directly on the device using FFmpeg http://ffmpeg.org/

FFmpeg is a set of open source libraries that allow you to record convert digital audio and video recordings in various formats. It includes libavcodec a library for encoding and decoding audio and video and libavformat a library for multiplexing and demultiplexing into a media container. The name comes from the name of the MPEG and FF expert group meaning fast forward.
FFmpeg is already built into the program and does not require downloading additional codecs.
The conversion takes place directly on the device (the Internet is not required) and the conversion speed depends on the processor speed of the device.

Supports: MPEG4 h265 h264 mp3 3gp aac ogg (vorbis and theora) opus vp8 vp9 and many other formats (you will find the list in the app).

Requirements: Android 4.4 and the availability of the processor ARMv7 ARMv8 x86 x86_64.

FFmpeg with x264 x265 ogg vorbis theora opus vp8 vp9 mp3lame libxvid libfdk_aac libvo_amrwbenc libopencore-amr speex libsox libwavpack libwebp librtmp

More options can be found in the help pages for FFmpeg.

For Android 11 users: New rules require the application to use more confidential methods of working with files on your device. You will have to copy/move the input files to a shared folder such as DCIM Movie Music Download. Sorry for the inconvenience