wav file audio samples are signed, how does DAC deal with signed numbers when it does not have negative supply?
Lets say you have a DAC with a reference of 2.5V operating from a single supply (i.e. the full span output is 2.5V) that accepts 2s complement encoded values at the digital input.

The zero point in such a device is at \$\frac {V_{span}} 2\$ or 1.25V in this particular application.

If a device takes 2s complement as an input, then the values below 0 are encoded as less than the mid-span voltage; specifically, \$V_{out} = V_{midspan} + Code_{in} * V_{bit}\$ where \$V_{bit}\$ is the resolution; \$ V_{bit} = \frac {V_{span}} {2^n}\$ where n is the number of bits for the converter. 

\$V_{code}\$ is the signed value of the input.

Note that on a split supply (+/- 2.5V for example) midspan is 0V.

There is an excellent application note that explains the various types of data formats used by DACs and ADCs.

Floating point numbers would normally be converted to integers within the range of the converter for a DAC. ADCs universally (all the ones I have seen) output fixed point values.

There are complex devices that have DSPs inside that can do the floating point to fixed point conversion on board.