Hi
What you describe normally happens, when audio files are played back on hardware with a mismatched sample rate. So I suggest to go thru all places on your system where a sample rate can be selected and check the settings. How are the clock settings for your Fireface UC? Do you have an external unit (on SPDIF or ADAT) set as master clock?

Using digital audio devices, they have to be synced to properly exchange the audio data. Because not every clock generator has the exact same speed (e.g. one is on 44'109 and the other on 44'096), there could be clicks and pops hearable when they aren’t synced to each other. For this, one clock has to be the 'master' and sends its 'beat' to all the other invloved units which are 'slaves' - they don’t use the internal generator anymore but take the clock from the 'master' unit (sadly 'master' and 'slave' are the common terms - I don’t like it). So now all the units with their AD or DA converters are in sync.
If you have an interface locked to e.g. an external AD converter on 48kHz over SPDIF and you play some audio files which have a sample rate of 44.1kHz, they would sound exactly like what you described because the samples are converted too fast. In RME Fireface Settings, what settings do you have there regarding word clock / sample rate?