Q: no stereo - only one speaker on 2009 Mac Book Pro 15"

The obvious question, have you checked the balance in System Preferences>Sound?

Try a NVRAM reset:

https://support.apple.com/en-us/HT204063

It could be that the speaker connection needs adjustment or the speaker is faulty.

Ciao.

1. System Preferences > Sound > Output > Internal Speakers/Headphone

       Settings for the selected device:

       Balance:

       Make sure that the slider is set at middle.

2.  AudioMIDI Setup

       Applications/Utilities/ Audio MIDI Setup.app

       Audio Devices window

       Side Bar

       Click the Built-in Output.

       Make sure that CH1 and CH2 volume slider settings are matched.

3. Reset SMC.

      Reset SMC.      https://support.apple.com/en-us/HT201295