How can I connect two audio channels (L+ L- and R+ R-) to a jack (L, R, GND)?
You may well find that using L+, R+ (via AC coupling, e.g. 100uF capacitors) and GND works.
Depending on the internal details of the module, L+/R+ are likely to be accurate copies of L,R referenced to GND (but with a Vcc/2 bias) while L-,R- are inverted copies.
In that case, there is no harm in leaving L-,R- disconnected.
Quality may depend on how clean the Vcc supply to that device is; you may need to add decoupling to the supply you are giving the module.
Some more info here regarding a similar device :
Cost Effective Solution to using PAM8403 as a headphone amp