Soundclouds API is returning 403 on some tracks for me. I have tried playing with the raw http endpoints and also the soundcloud api wrapper for python, both have the issue.
Using your two examples I get valid results for both
Example 1:
https://api.soundcloud.com/resolve?url=https://soundcloud.com/calyxteebee/nothing-left&client_id=CLIENT_ID
returns
https://api.soundcloud.com/tracks/251164884?client_id=CLIENT_ID
Example 2:
https://api.soundcloud.com/resolve?url=https://soundcloud.com/mtarecords/my-nu-leng-flava-d-soul-shake&client_id=CLIENT_ID
returns
https://api.soundcloud.com/tracks/249638630?client_id=CLIENT_ID