Hi @jusse,
I’m wondering if the headphone jack is faulty and needs to be replaced.
Usually when headphones are plugged in, depending on the type of jack used and the circuit design, it either mechanically cuts off the audio to the laptop’s speakers and redirects it to the headphones or an internal contact in the jack sends a signal to the audio controller to switch the audio away from the speakers and onto the headphones.
Here are some guides that may be of some help.
[[Topic:Toshiba Satellite C55Dt-A5306]] guide
[guide|99884] guide
[https://www.ebay.com/itm/New-Toshiba-Satellite-C55-A-Audio-Jack-Headphone-Microphone-Socket-Connector-Gen/192229002303?hash=item2cc1bdcc3f:g:Up4AAOSwveRbiLde|Headphone socket - (supplier example only)]. Just search online for ''Satellite C55D Audio jack'' to get suppliers that suit. I realize that the example only says C55 but couldn’t find any that specifically said C55D or even C55A or C55B etc so maybe it is the same for all C55 models