there are a couple of options missing:
sr = 44100
nchnls = 2
0dbfs = 1
put them inside CsInstruments, before any instrument.
The most important is 0dbfs, telling csound what is the max volume. The default, for historical reason, is 32768 (you can see in the csound output “0dBFS level = 32768.0”). Without that line, vco2 volume is 1/32000 of the max volume.
Also, setting nchnls to 2 makes it stereo, so you also have to change “out aOut” to “outs aOut, aOut”. This should work, both with -odac and without:
<CsoundSynthesizer>
<CsInstruments>
sr = 44100
nchnls = 2
0dbfs = 1
instr 1
aOut vco2 1, p4
outs aOut, aOut
endin
</CsInstruments>
<CsScore>
i1 0 1 100
i1 1 1 200
i1 2 1 300
</CsScore>
</CsoundSynthesizer>