A critical issue has been found in uTool (184.108.40.206) when used in MAC.
When the source mono wave files for creating a multitrack song are not of the same length, the uTool appends silence so that all the tracks are of equal length. Unfortunately with the MAC version of uTool 220.127.116.11, with the shorter tracks, instead of silence, a part of the track might be appended in the end.
We are working on this bug with the highest priority and we plan to release a fix as soon as possible.
Until the bug is fixed, you may want to use a standard DAW to convert your mono wave files to have the same length.
Note: The problem does not occur in the windows version of uTool.