I don't know which engine sound you have found, so I can't be specific about how to swap that one for your Subaru 2005 in RSRBR2010.
However, I have swapped sound files from one car to another, and here is how to do it.
Please note: This involves changing data within RSRBR2010, so don't attempt it if you are not familiar with altering files manually. Also, make sure you create Backups of relevant files in case you have a problem.
First up, RSRBR2010 changes files from the default game each time you run it. So you need to make the changes within RSRBR2010.
In the following, I will give you the example of swapping the Subaru wrc 2003 engine sound to the Subaru wrc 2005.
1. First, find out which engine sound file is used for the Subaru wrc 2003 (the sound you want to use).
2. Go to your RBR/RSRBR2010 folder and then go to Sounds.Sav/Engines folder.
3. Open A8W-03.ini using Notepad.
4. Look for the Subaru Impreza 2003 lines ( should be car 6). The second line will show which Engine file is used (Car6=WRC_subaru_03a)
Now, to change the Car you want to drive:
5. Open A8W-05.ini (Make a Backup first)
6. Look for the Subaru lines (should be Car 0). The second line will have the engine file used for this car (Subaru_Impreza_03_c1)
7. You need to change this engine file to the one from the Subaru 2003.
8. Change the line Car0=Subaru_Impreza_03_c1 to Car0=WRC_subaru_03a.
9. Save the new file. You should now have the Subaru 2003 engine in the Subaru 2005.
Sorry for the long explanation, but I wanted you to understand how it works, so that you can swap other engines if you ever want to.