Usually it plays the common audio formats MP3, AAC, MP4, OGG, OPUS, FLAC, WAV. As 1by1 uses the system decoders, the supported formats depend on the device, usually they should all be present. On many devices also MP2 is supported. Older devices may not decode OPUS - or only if the files have the OGG extension. On MP4 only the audio track will be played. M4A is also supported.

A newly started blank 1by1 shows a root folder without any tracks. But they are right around the corner. If the music is on the device's storage, storage/emulated/0/Music may be the location. You also could reach your files under storage/emulated/0 or mnt/sdcard. If the audios are on a removable media (microSD), they should be under something like storage/1234-5678. Be sure to accept the storage permission, otherwise nothing is shown. Once you started a track and quit 1by1, it will remember the location and show it directly on start.

Folders can be sorted by name, date, month or be shuffled. Month means that all tracks of a season can be together regardless from which year they are. Unfortunately some Android versions destroy the file date on copying. M3U playlists can be sorted by name, path, original order and also be shuffled.

Audio enhancing, normalize, mono-mix, and transition mixing need 'Internal decoding' enabled. You find this switch in the settings. To (un)check it, play must be stopped before. Below you can enable and adjust the DSP features. To quickly toggle enhancer and mono-mix, you can also use the Sort menu.

Yes, it can. The URLs must be listed in an M3U playlist file - and directly point to the stream. So it may be necessary to find the stream URL before. Sometimes it must be extracted from a PLS or M3U playlist file. 1by1 can play different streams from one M3U. To show the stream name instead of just the URL, also the #EXTINF name above the URL is shown within 1by1.