Interior bifold doors are normally hollow and this allows sound to easily pass through the doors. This may not be a major issue, however if you have a noisy dog living next door or people talking loudly then you might want to consider using sound proofing material inside the door.
Conversations and noises can be carried through the door itself and through any gaps around the door. If you are trying to prevent hearing any other noises and want to be more private then you should use soundproofing. Soundproofing bifold doors isn't even that difficult if you have the right tools and materials available.
Step 1 - Checking for Gaps
If the bifold doors don't fit perfectly into the opening in the wall then this will allow sound and heat to escape. You need to look for any gaps which are around the door frame. If there are any gaps around the door, no matter how small they can allow a lot of sound and heat to escape. If there are any gaps you should use closed cell tape to prevent them. This tape is very cheap and affordable.
Step 2 - Bottom of Door
If there is a gap at the bottom of the door then this is also very important to correct. Fit a small draft excluder to the bottom of the door, you may have to cut this into small sections so that it will allow the doors to fold back on themselves. The draft excluders need to be screwed into the bottom of the door so that all of the gaps are hidden. Only the flexible rubber or brush part of the draft excluder should touch the door frame because this will then stop sound waves from escaping through the holes.
Step 3 - Checking Insulation
Take a look at whether there is any insulation between the door and the wall. Take a look at this by removing the molding around the base of the door, do this carefully so that you don't cause any damage to the decor of the door. Take a look inside the door and find out whether or not there is already insulation inside.
Step 4 - Insulation
Put caulking into the cavity in your door to provide more insulation and to prevent sound waves from passing through the door. Additional insulation material material can be placed inside the door.
Step 5 - Additional Insulation
If you still feel that your bifold door needs more insulation then you should consider adding additional insulation to the outside of your door. This can be done by using a quilt which can be stuck to the door using velcro strips. Attaching a quilt will deaden almost all sound from within or outside a room.
Learning how to sound-proof bifold doors can make them much more practical. although you might not be interested in using a quilt, you can benefit by using internal insulation.