

Chose which works best for you in the Reaper Sound Card settings. ASIO is more of a professionally used driver, but it is limited to 1 app at a time accessing the sound card. Microsoft Sound Mapper is commonly selected by default in other apps because it is most compatible when audio is played from multiple apps, but also provides the worst audio latency problems with the specific app that is using it. Something like WDM, WAVE and Microsoft Sound Mapper drivers don't have this same problem of sharing audio between apps and is likely what the other apps have selected by default. If you want a quick fix, then go into the audio device options inside of Reaper and select a different sound card driver for the UMC-404HD. Which driver are these "other apps" which don't have this problem using? This is a very common problem with using ASIO drivers, which Reaper will select by default since it provides the best latency performance.
