Rivendell can sometimes incorrectly calculate the length of a file, meaning that it cuts off early. This generally happens when your file has been saved as a Variable Bit Rate (VBR) MP3 file. You can avoid this by ensuring the file has been saved as a Constant Bit Rate (CBR) MP3.
Saving as a CBR file in Audacity
In the Export Audio dialog, under Format Options, ensure Constant is selected with a quality of 256 kbps
Saving as a CBR file in Reaper
On the Render to file... dialog, in the Primary output format section, select MP3 and set Constant bitrate and 256 kbps bitrate
But it works fine when I play it at home... (aka the Technical Bit)
There are basically two types of MP3 file: Constant Bitrate (CBR) and Variable Bitrate (VBR). As the names suggest, CBR files supply a constant amount of data for each second of the track, whereas VBRs change the amount of data per second depending on the audio. As a result, VBRs are more efficient, but it's much harder for software to calculate the file length.
Most domestic music software deals with VBRs by analysing the file in its entirety when you load it, to work out the file length. Rivendell is much more naïve and uses the mean average file bitrate to calculate the length, making loading the file faster but the calculation less precise. This means that the calculated length can be several seconds more or less than the real length. If it is calculated as less than the real length, the file will get cut-off early.
The only way around this is to ensure you use a Constant Bitrate when you save the MP3.