Hi Lemming,
the answer is in the TPA3116D2 datasheet at paragraph 7.3.7, page 16:
To use the TPA31xxD2 family with a single-ended source, ac ground the negative input through a capacitor equal in value to the input capacitor on positive and apply the audio source to either input. In a single-ended input application, the unused input should be ac grounded at the audio source instead of at the device input for best noise performance. For good transient performance, the impedance seen at each of the two differential inputs should be the same.
Regards
Marco