1) Samsung Galaxy S2 - Approx 28k onwards depending on online promo, stores
Gaming - Incredible Gaming performance thanks to Dual core and Mali 400 GPU. Can handle any 3d intensive games with ease
Music - Decent but not up to par with Iphone 4 or Galaxy S. Iphone 4 and Galaxy S is considered to have top audio capability
2) Sony Ericsson Xperia Arc - Approx 26k onwards
Gaming - Not comparable to dual core phones. Decent gaming performance but still shows weakness since Adrenos 205 GPU is not on par with Powervr SGX 540 (Found in Galaxy and Nexus S)
Music - Music will be good since it is an Sony. While I personally feel it as not better than Galaxy S or Iphone 4.
3) Sony Xperia Play - 29k onwards
Gaming - While Adrenos 205 is decent as mentioned above, this phone has official playstation game controls. Also despite having only Adrenos 205 gpu, it still will get many games thanks to Sony and also PS phone.
Music - Since it is sony the music experience will be good but once again as mentioned above, not comparable to Galaxy S or Iphone 4.
4) Apple Iphone 4 - 30k onwards
Gaming - While it has the least powerful gpu of the bunch, it still can play decent games as Apple provides games that is optimized for Iphone 4 and its GPU. So gaming experience will be good.
Music - Since it is an Apple, it has music player that rivals the best of em due to Wolfson chip
5) HTC Sensation - 28k onwards
Gaming - Once again this is a Dual core phone with Adrenos 220 GPU. So guess what, Gaming will be excellent with this phone. Also it has been proven that Adrenos 220 is almost on par or slightly better than Mali 400 GPU for which I will provide link:
Difference Between Mali-400MP GPU and Adreno 220 GPU | Difference Between | Mali-400MP GPU vs Adreno 220 GPU
Music - Once again music is decent but falls short when compared to Iphone 4 or Galaxy S. Even S2 seems better than Sensation IMHO.
Since music is a main requirement and occasional gaming, from your list I would choose either Galaxy S2 or Iphone 4.
Here are other models that maybe you can look at, if you are interested:
Galaxy S - 19k
Gaming - Occasional gaming or even heavy gaming, this phone still has the best gpu for any single core phones. With overclocking my processor and in turn increasing my GPU memory, the GPU plays games like Shadow of Guardian, Splinter Cell and other games with ease. Benchmarking I get gpu/gaming scores identical to dual core phones.
Music - Wolfson DAC which will provide Digital Dolby 5.1 surround sound that is available not only for music, but any movies you play. So it makes it the best audio phone for me personally.
Nexus S - 19k
Same as Galaxy S. Only thing missing is Dolby Digital 5.1 but still has the same Wolfson DAC and better audio.