Your best option is to set up a Monitored Folder on the Dropbox folder which will update the track in PlayIt Live when the file changes in Dropbox.
Personally I don't use Dropbox much but OneDrive is capable of having a folder on the PC which is kept in sync with the "cloud" version.
Assuming that Dropbox has a similar client then using that and pointing PlayIt Live at that folder and file seems worth a go.
Sorry, I assumed that you had the Dropbox client installed that synchronised your files to your computer. You can select the folders you want if you only want to download certain folders.