Effects - Tempo -confusion

Have had some demos made in a Nashville studio using whatever “professional” software they use (mostly Belmont College trained guys). Several times the tempo seemed sluggish so changed it in Audacity. (ie 60 BPM to approx 68). Asked the recording studio to please just change tempo on some of the songs (when they do the final mix and mastered version) using their software and they claim it cannot be done without affecting the tones and quality of the various tracts. Guess I need to know if their statements are true and how to prove them incorrect if they are incorrect. Really looking for some technical proof type statement. Thanks

Yes there will be some change in timbre.
How much change, and whether it is acceptable or not depends on the tempo change algorithm and the type of material it is being applied to.

Here is an example that plays 4 times. Other than the first version, the tempo has been increased by 13.333% (equivalent to a change of 60 bpm to 68 bpm)

  1. Original
  2. Audacity “Change Tempo” effect, “High Quality” algorithm off
  3. Audacity “Change Tempo” effect, “High Quality” algorithm enabled
  4. Audacity “Change Speed” effect. (Changes tempo and pitch).

My assessment of these examples (you may not agree)
The second version is possibly a little muffled, but to my ear sounds is the most acceptable of the speeded up versions.
The third version seem to retain the high frequencies better, but the stereo spacial effect swooshes around (most noticeable on headphones), and sounds quite weird.
The “losses” are least in version 4, but the change in pitch is extremely obvious and probably not acceptable.